High Stream Power Research Articles

Morphological Variations of A Certain Tigris River Reach For Different Periods in Iraq

A morphological variations occurs in the paths of rivers due to many reasons such as the hydraulic changes in the natural flow regime of rivers in-addition to the human activities on rivers such as dams constructions on rivers. Many morphological variations created on Tigris river reach within Mosul city, north Iraq for different periods between 1956 to 2002 had been studied such as the changes on the river width specially near the bridges sites and also the development of the dimensions of the islands and bars within the river reach. The available topographic maps and also an aerial photographs were depended during the data analysis with some field reconnaissance . The research work concluded that one of the main reasons for the development of the islands is the flow regime variation of Tigris river as a result of the Mosul dam operation which in-turn leaded to a reduction in the transported sediment load in the reach due to the released high stream power clear water discharges of sediment from Mosul dam. The second reason was the reduction in the water level which in-turn change the middle and side islands in the river with the growing vegetation cover such as trees and weeds specially in the sites downstream the meander of Al-Rashidia Village and downstream Al-Shuhadaa bridge and upstream Ninevah bridge and finally upstream the fourth bridge in Mosul city.

Transient landscape evolution of basement‐cored uplifts: Example of the Kyrgyz Range, Tian Shan

The Kyrgyz Range, located on the northern margin of the western Tian Shan, illustrates long‐term (106–107 year) transient landscape evolution in response to an active basement‐cored rock uplift. Eastward propagation of range uplift has progressively exposed resistant bedrock capped by a tilted, (formerly) planar, pre‐Cenozoic unconformity. We develop an approximate, stream power–based analytic model of transient river profile incision into progressively exposed and resistant bedrock to explore the patterns of channel development into the unconformity surface. This analysis shows that the unconformity can be preserved as a geomorphic marker defined by upland headwaters and interfluvial areas. Though channels are not at equilibrium with rock uplift, prominent knickpoints are not predicted to develop on main stem channels. However, knickpoints are predicted to develop on tributaries upstream of their junctions with the trunk stream because of differential erosion rates. Initial channel slope and transient channel form are both sensitive to the n value of the stream power model and could prove useful for calibration of n from field data. Accumulation of catchment area (a proxy for discharge) into larger catchments develops a positive feedback where larger drainages with higher stream power at a given slope undermine and capture adjacent drainage area. A simple model of competitive fluvial incision illustrates the role of tributary junction position in maximizing stream power expended upon the trunk stream. Examples from the Kyrgyz Range illustrate the effects of tributary junction position on fluvial relief, and we propose that adjustments to the tributary network through stream capture are ongoing within this landscape even after several kilometers of exhumation.

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

The Yarlung Tsangpo–Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo–Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO 2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo–Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km −2 yr −1 and the silicate cation flux more than 34 tons km −2 yr −1. In total, chemical weathering in this area consumes 15.2 × 10 5 mol CO 2 km −2 yr −1, which is twice the Brahmaputra average. These data show that 15–20% of the total CO 2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na + and Cl − fluxes throughout southern Tibet, comprising more than 50% of all Na + in some stream systems. Carbonate weathering generates 80–90% of all dissolved Ca 2+ and Mg 2+ cations in much of the Yarlung Tsangpo catchment.

Evolution of Quaternary alluvial fans and terraces in the intramontane Pinjaur Dun, Sub‐Himalaya, NW India: interaction between tectonics and climate change

AbstractQuaternary alluvial fans in the tectonically active Pinjaur Dun, an intramontane valley in the Sub‐Himalaya, were deposited in front of the Nalagarh Thrust and were influenced both by tectonics and glacial climate fluctuations. The surface morphology indicates that an earlier set of first‐order fans (Qf1) became entrenched and onlapped by a series of second‐order fans (Qf2). The younger fan segments were then cut by a pair of terraces (T1 and T2). Quartz optically stimulated luminescence dating establishes that the Qf1 aggradational phase was initiated before 96·5 ± 25·3 ka and terminated after 83·7 ± 16·3 ka. This was followed by a period of incision, before Qf2 fan deposition started at 72·4 ± 13·4 ka and continued until 24·5 ± 4·9 ka. Sediment was deposited on the T1 (upper) and T2 (lower) terraces at 16·3 ± 2·1 and 4·5 ka, respectively, recording a return to overall degradation punctuated by minor deposition on terraces. The period of incision separating the younger and older fan deposits coincided with enhanced SW monsoon precipitation. The subsequent development of the Qf2 fans and their progradation until 20 ka suggest erosional unloading of the thrust hangingwall during a tectonically quiescent phase. Toe cutting, deposition of axial river and lacustrine facies, and retreat of Qf2 around 45 ka, indicate fanward shift of the axial river due to tilting of the valley towards the NE in response to reactivation of the Nalagarh Thrust. The cessation of Qf2 deposition around 20 ka and the onset of through‐fan entrenchment suggest reduced sediment supply but relatively high stream power during the last glacial maxima (LGM). The prolonged stream incision since the cessation of Qf2 deposition, with only minor depositional phases at 16·3 ± 2·1 and 4·5 ka, resulted from high water discharge and low sediment input during intensification of the SW monsoon and vegetation changes in the hinterland.

Stream channel convexity induced by continuous base level lowering, the Dead Sea, Israel

The Dead Sea level has dropped from − 388.4 m in 1896 to − 417.2 m in 2004 and continues to fall rapidly, reaching a record of 1 m/y drop. Such a dramatic base level fall, on time scales of several tens of years, makes the Dead Sea area a unique field laboratory. The aim of this study is to examine the effect of the lowering Dead Sea, as a base level, on the longitudinal profiles of channels, on the formation of knick points, and on the sinuosity. Field studies were made along the lower courses of creeks which drain lakeward. Precise longitudinal profile measurements and their fitting to linear and logarithmic curves, show they are planar to slightly convex. The short convexity, the absence of hydrological or lithological control, and former experimental results of profile response to base level fall, suggest that the convexity results from the recent continuous rapid drop of the Dead Sea. The convexity is also an inheritance of the paleo morphology, i.e., the original convex front of the fan deltas and coastal scarps following channel incision, slope displacement and diffusion. The erodible substrate and the high stream power prevented preservation of nickpoints along the main channels. Sinuosity of the meandering thalwegs remained low 1.1–1.2, indicating no response to the base level lowering along the steep slopes. Although the stream channels are incised in soft sediments, the base level-induced entrenchment dies out within 1000 m or less from the receding shoreline. The channel profiles demonstrate different phases of adjustment to the lowering base level. The uniqueness of the Dead Sea case is that, contrary to former field and experimental studies, the system is in a continuous relaxation period without reaching a final adjustment. The channels are “trapped” in their convex form following a rapid on going base level fall upon exposed paleo fan deltas.

Sediment rating parameters and their implications: Yangtze River, China

This study examines the characteristics of sediment rating parameters recorded at various gauging stations in the Yangtze Basin in relation to their controls. Our findings indicate that the parameters are associated with river channel morphology of the selected reaches. High b-values (> 1.600) and low log( a) values (< − 4.000) occur in the upper course of the steep rock-confined river, characterizing high unit stream power flows. Low b-values (< 0.900) and high log( a) values (> − 1.000) occur in the middle and lower Yangtze River associated with meandering reaches over low gradients, and can be taken to imply aggradation in these reaches with low stream power. Higher b-values (0.900–1.600) and lower log( a)-values (− 4.000 to − 1.000) characterize the reaches between Yichang and Xinchang, immediately below the Three Gorges. These values indicate channel erosion and bed instability that result from changes in channel gradient from the upstream steep valley to downstream low slope flood plain settings. Differences in channel morphology accompany these changes. Confined, V-shaped valleys occur upstream and are replaced downstream by broad U-shaped channels. The middle and lower Yangtze shows an apparent increase in channel instability over the past 40 years. This inference is based on sediment rating parameters from various gauging stations that record increasing b-values against decreasing log( a)-values over that time. Analysis of the sediment load data also reveals a strong correlation between changes in sediment rating curve parameters and reduction of annual sediment budget (4.70 × 10 8 t to 3.50 × 10 8 t/year, from the 1950s to 1990s), largely due to the damming of the Yangtze and sediment load depletion through siltation in the Dongting Lake. Short-term deviations from the general trends in the sediment rating parameters are related to hydroclimatic events. Extreme low b-values and high log( a)-values signify the major flood years, while the reverse indicates drought events. When compared with rivers from other climate settings, it is evident that the wide range of values of the Yangtze rating parameters reflects the huge discharge driven by the monsoon precipitation regime of eastern China.

Comparative geomorphic analysis of surficial deposits at three central Appalachian watersheds: Implications for controls on sediment-transport efficiency

Factors that control the routing and storage of sediments in the Appalachian region are poorly understood. This study involves a comparative geomorphic analysis of three watersheds underlain by sandstones and shales of the Acadian clastic wedge. These areas include the Fernow Experimental Forest, Tucker County, West Virginia; the North Fork basin, Pocahontas County, West Virginia; and the Little River basin, Augusta County, Virginia. GIS-based analyses of surficial map units allow first-order approximation of sediment-storage volumes in valley bottoms. Estimates of volumes are examined in tandem with morphometric analyses and the distribution of bedrock channels to make inferences regarding controls on sediment-transport efficiency in the central Appalachians. The Fernow and North Fork areas are characterized by V-shaped valleys with mixed reaches of alluvial-bedrock channels distributed throughout the drainage network. In contrast, the Little River valley is notably wider and gravelly alluvial fill is abundant. Comparator watershed parameters for the Fernow, North Fork and Little River areas include, respectively: (1) basin area = 15.2 km 2, 49.3 km 2, 41.5 km 2; (2) basin relief = 0.586 km, 0.533 km, 0.828 km; (3) drainage density = 4.2 km − 1 , 3.3 km − 1 , 4.7 km − 1 ; (4) ruggedness = 2.5, 1.7, 3.9; (5) Shreve magnitude = 139, 287, 380; (6) total valley-bottom area (km 2) = 0.76 km 2, 1.86 km 2, 3.09 km 2; (7) average hillslope gradients = 17.2°, 18.4°, 22.1°; (8) total debris-fan surface area = 0.113 km 2, 0.165 km 2, 0.486 km 2; and (9) debris-fan frequency = 2.0 km − 2 , 1.0 km − 2 , 2.8 km − 2 . The storage volumes in valley bottoms were estimated using map polygon areas and surface heights above channel grade. The Little River contains significantly higher sediment volumes in floodplain, terrace and fan storage compartments; total volumes of the valley bottoms are approximately twice that of the Fernow and North Fork areas combined. Unit storage volumes for the Fernow, North Fork and Little River are 5.2 × 10 4 m 3 km − 2 , 5.5 × 10 4 m 3 km − 2 and 1.6 × 10 5 m 3 km − 2 , respectively. A conceptual model postulates that valley-width morphometry and style of delivery from hillslopes are the primary factors controlling the efficiency of sediment transport. Steep, debris-flow-prone hillslopes at the Little River deliver high volumes of gravelly sediment at magnitudes greater than transport capacity of the channel. Patterns of stream power are complex, as low-order tributaries are under capacity and high-order tributaries over capacity with respect to sediment load. Aggraded alluvial fill insulates valley-floor bedrock from vertical erosion and valley widening dominates. Expansion of the valley width creates a positive response via increased storage capacity and lower unit stream power. Conversely, the Fernow and North Fork are characterized by diffusive mass movement on hillslopes with incremental bedload transport to higher-order tributaries. Rates of hillslope delivery are balanced by the rate of channel export. Mixed alluvial-bedrock reaches provide the optimal channel configuration for active incision of the valley floor. Low expansion of valley width promotes high unit stream power and processes of vertical erosion. The model implies that the Fernow and North Fork have been more effective at sediment transport during the Late Quaternary. Given similar climatic and tectonic settings, variation in bedrock lithofacies is likely the primary factor modulating the efficiency of sediment transport.

Wood storage in a wide mountain river: case study of the Czarny Dunajec, Polish Carpathians

AbstractStorage of large woody debris in the wide, mountain, Czarny Dunajec River, southern Poland, was investigated following two floods of June and July 2001 with a seven‐year frequency. Within a reach, to which wood was delivered only by bank erosion and transport from upstream, wood quantities were estimated for eighty‐nine, 100 m long, channel segments grouped into nine sections of similar morphology. Results from regression analysis indicated the quantity of stored wood to be directly related to the length of eroded, wooded banks and river width, and inversely related to unit stream power at the flood peak. The largest quantities of wood (up to 33 t ha−1) were stored in wide, multi‐thread river sections. Here, the relatively low transporting ability of the river facilitated deposition of transported wood while a considerable length of eroded channel and island banks resulted in a large number of trees delivered from the local riparian forest. In these sections, a few morphological and ecological situations led to the accumulation of especially large quantities of wood within a small river area. Very low amounts of wood were stored in narrow, single‐thread sections of regulated or bedrock channel. High stream power facilitated transport of wood through these sections while the high strength of the banks and low channel sinuosity prevented bank retreat and delivery of trees to the channel.Considerable differences in the character of deposited wood existed between wide, multi‐thread channel sections located at different distances below a narrow, 7 km long, channellized reach of the river. Wood deposited close to the downstream end of the channellized reach was highly disintegrated and structured into jams, whereas further downstream well preserved shrubs and trees prevailed. This apparently reflects differences in the distance of wood transport and shows that in a mountain river wider than the height of trees growing on its banks, wood can be transported long distances along relatively narrow, single‐thread reaches but is preferentially deposited in wide, multi‐thread reaches. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Geomorphic characterization and diversity of the fluvial systems of the Gangetic Plains

The extensive Gangetic alluvial plains are drained by rivers which differ strongly in terms of hydrological and sediment transport characteristics. These differences are manifested in the geomorphic diversity of the plains. The Western Gangetic Plains (WGP) are marked by a degradational topography with incised channels and extensive badland development in some parts, while the Eastern Gangetic Plains (EGP) are characterized by shallow, aggrading channels with frequent avulsions and extensive flooding. We interpret such geomorphic diversity in terms of differences in stream power and sediment supply from the catchment areas. The rivers draining the western plains are marked by higher stream power and lower sediment yield that result in degradation. In comparison, the rivers draining the eastern Gangetic Plains have lower stream power and higher sediment yield that result in aggradation. The variation of stream power, a function of channel slope and high sediment yield, is attributed to differences in rainfall and rate of uplift in the hinterland. It is suggested that such differences have resulted in a marked geomorphic diversity across the plains. It is also suggested that such diversity has existed for a fairly long time because of climatic and tectonic variance.

Flow events on a hyper-arid alluvial fan: Quebrada Tambores, Salar de Atacama, northern Chile

Abstract The Tambores alluvial fan is located within the hyper-arid Atacama Desert of northern Chile. We examine evidence of the range of flow processes operative in this environment from a combination of Pleistocene-Holocene fan deposits and a recent (2001) flood event (16 m 3 s −1 ) in the fan feeder channel and upper alluvial-fan area. The field evidence suggests that peak flows recorded in the older deposits generated extensive sheetflood events dominated by antidune deposition in the upper fan area. These extreme, supercritical flows were generated by floods with sustained high sediment and water discharges and high stream power. Easily erodable alluvial source materials ensured high sediment discharge could be maintained within flood events. High stream power was ensured as a function of the tectonically exacerbated gradients within the source area. The 2001 event indicates the rapid rheological changes that can occur within an individual flood event, ranging from hyperconcentrated streamflow to mudflow. The flow deposits vary little in maximum clast size either between the varying flood events in the upper fan area, or down the fan gradient. This is due to a limited calibre of sediment being produced from the source area. The study highlights: (1) the range of flow rheologies that can be generated from a hyper-arid catchment both within and between flood events of varying magnitude and the associated difficulties in generating a reliable stratigraphy from the resultant deposits; (2) the high stream power and sediment discharge associated with major flood events and thus the nature of flood hazard in the catchment and on the fan; and (3) the limitations of sedimentological information such as maximum clast size as an indicator of peak flow characteristics in ancient deposits.

A conceptual model of the role of excess energy in the maintenance of a riffle–pool sequence

There is much debate over the role of hydraulic ‘reversal’ in the maintenance of riffle–pool sequences in gravel-bed channels. It is, however, generally acknowledged that the pool has the greatest maximum energy during flood flows allowing scour and pool maintenance. Little emphasis has been placed on the reversal period in many previous studies despite its potential importance being noted in the seminal study of Keller [Geol. Soc. Am. Bull. 82 (1971) 753]. This is potentially significant, as reversal may only occur for short periods at the peak of flood hydrographs; hence, the hydraulic argument behind riffle–pool maintenance is based around what is often a marginal energy excess in the pool for a very short period of time. This study uses the magnitude frequency concept of Wolman and Miller [J. Geol. 68 (1960) 54] and applies it to an investigation of the temporal dominance in morphologic unit stream power between a series of four pools and three riffles derived from measured flow and calculated energy slopes on the upland gravel-bed River Rede, Northumberland. The geomorphologically effective flow regime is used to define temporal balances between riffle and pool energy levels. The geomorphic work carried out by riffles and pools, as described by the excess stream power ( Ω− Ω c) capable of transporting the bed sediments, displayed a broad balance for the upstream four units before reducing significantly at pool 3 and rising again for the subsequent two units which appear particularly energetic. These findings contrast with hydraulic theories of riffle–pool maintenance, e.g., Keller [Geol. Soc. Am. Bull. 82 (1971) 753]. For sediments to be transferred through a riffle–pool sequence whilst maintaining a quasi-equilibrium form, the pools should carry out the same amount of work as the riffle upstream in order for it not to fill with sediment. Although stream power reversal did occur for some riffle–pool units, the magnitude and longevity of the reversal did not balance the energy exerted at riffle locations. Armouring and shallow gravel depth at riffle 3 appear to indicate scouring in response to high excess stream power found in this area. However, there was no evidence of sub-dominant pools 3 and 4 filling in response to low stream power.

Downstream grain-size distribution of surficial bed material and its hydro-geomorphological significance in a large and regulated river: the Rhône River in its delta area (France) / Distribution granulométrique longitudinale des dépôts de fond superficiels et sa signification hydro-géomorphologique dans un grand fleuve chenalisé : le Rhône dans son aire deltaïque (France)

Abstract This paper characterises the bed material in the two regulated distributaries of the Rhône Delta and examines the relationships among grain size, channel geometry, hydraulic conditions and distance from the coast. The Rhône River in its delta area is characterised by four bed- material types (cobble-pebble, sand, compact silt, mud). Sand beds are the most frequent. During low water flow, a small proportion of sand grains is mobilised by rolling. Sand transport capacity strongly increases at bankfull discharge, and bedload transport of sand is replaced by graded suspension. Finest cobble-pebble beds of the Grand Rhône are strictly mobile at bankfull discharge. Bed-material grain size is distinct between the Grand Rhône, which mainly transports cobble-pebble and coarse to medium sand, and the Petit Rhône, which predominantly transports medium to fine sand. Specific stream power differences, combined with local structural and geomorphological factors, explain this distinct bed-material grain size distribution in the two channels. An abrupt transition from cobble-pebble to sand beds occurs in the Grand Rhône 25 km upstream from the river mouth. On the Petit Rhône, a downstream fining occurs in the sand beds located in the lower section. At present, the Grand Rhône has a higher competence and a higher bed-material transport capacity than the Petit Rhône. High specific stream power explains the presence of numerous incised channel sections that create a bed-material discontinuum.

River incision or diversion in response to bedrock uplift

INTRODUCTIONMany researchers have examined the long-term behavior of alluvialrivers (i.e., Schumm et al., 1987; Snow and Slingerland, 1990; Paola et al.,1992). On long time scales, alluvial rivers act as sediment-transport systemsthat broadly deposit sediment over low areas in their basins (i.e.,Tucker andSlingerland, 1996). The behavior of bedrock rivers is less well understood,although most workers agree that bedrock erosion is some function of streampower (Howard and Kerby, 1983; Seidl and Dietrich, 1992; Howard et al.,1994). Accordingly,researchers addressing a river’s response to bedrock up-lift equate incision with relatively high stream power and diversion with rela-tively low stream power (e.g., Burbank et al., 1996). However, these studiesassume that the erosion rate of the bedrock channel across the mountainrange controls the eventual outcome; they have not examined the behavior ofthe alluvial river upstream and downstream of the uplift. We propose to lookat the entire erosional and depositional river system and examine the long-term response of a coupled alluvial-bedrock river system to tectonic uplift.Our motivation is general and aims at a greater understanding offluvial and tectonic interactions, but our model is necessarily both specificand simplified:the tectonic uplift of a bedrock massif into the path of a later-ally unconstrained alluvial river. Such a scenario has two possible out-comes: either the river maintains its original course and incises through theuplift, or the river diverts and forms a new course around the uplift. A sig-nificant complication in the analysis of this scenario is that the evolvingriver system is partial alluvial and partial bedrock channeled. Because thebehaviors of these two river types are different, modeling requires couplingdiverse systems. We construct an analytical model to perform a first-orderanalysis of this coupled system. Our objective is to identify the specific fac-tors that determine the final outcome of river diversion or incision.Our model focuses on long time scales (10

Influence of storm-related sediment storage on the sediment delivery from tributary catchments in the upper Waipaoa River, New Zealand

Although much is known about overall sediment delivery ratios for catchments as components of sediment production and sediment yield, little is known about the component of temporary sediment storage. Sediment delivery ratios focused on the influence of storm-related sediment storage are measured at Matakonekone and Oil Springs tributaries of the Waipaoa River basin, east coast of New Zealand. The terrace deposits of both tributaries show abundant evidence of storm-related sedimentation, especially sediment delivered from Cyclone Bola, a 50 year return rainfall event which occurred in 1988. The sediment delivery ratio is calculated by dividing the volume of sediment transported from a tributary to the main stream by the volume of sediment generated at erosion sites in the tributary catchment. Because the sediment delivery volume is unknown, it can be calculated as the difference between sediment generation volume and sediment storage volume in the channel reach of the tributary. The volume of sediment generated from erosion sites in each tributary catchment was calculated from measurements made on aerial photographs dating from 1960 (1:44 000) and 1988 (1:27 000). The volume of sediment stored in the tributary can be calculated from measurements of cross-sections located along the tributary channel, which are accompanied by terrace deposits dated by counting annual growth rings of trees on terrace surfaces. Sediment delivery ratios are 0·93 for both Matakonekone catchment and Oil Springs catchment. Results indicate that Oil Springs catchment has contributed more than twice the volume of sediment to the Waipaoa River than the Matakonekone catchment (2·75 × 106 m3 vs 1·22 × 106 m3). Although large volumes of sediment are initially deposited during floods, subsequent smaller flows scour away much of these deposits. The sediment scouring rate from storage is 1·25 × 104 m3 a−1 for Matakonekone stream and 0·83 × 104 m3 a−1 for Oil Springs stream. Matakonekone and Oil Springs channels respond to extreme storms by instantaneously aggrading, then gradually excavating the temporarily stored sediment. Results from Matakonekone and Oil Springs streams suggest a mechanism by which event recurrence interval can strongly influence the magnitude of a geomorphic change. Matakonekone stream with its higher stream power is expected to excavate sediment deposits more rapidly and allow more rapid re-establishment of storage capacity. Copyright © 1999 John Wiley & Sons, Ltd.

Stream power influence on southern Californian riparian vegetation

Abstract. Mechanical damage by floodwaters is frequently invoked to explain the distribution of riparian plant species, but data have been lacking to relate vegetation to specific estimates of flood damage potential. This research uses detailed estimates of unit stream power (an appropriate measure of the potential for mechanical damage) in conjunction with vegetation cover data to test this relationship at 37 valley‐bottom sites in the Transverse Ranges of Southern California. A computer program, HEC‐2, was used to model the slope and the variation in flow depth and velocity of the 20‐yr flood across the sites. Regression models tested the influence of stream power (and of height above the water table) on the woody species composition of 393 4‐m cross‐section segments of the valley‐bottom sites. Results indicate that unit stream power does have a significant effect on the riparian vegetation, but that the amount of that influence and its importance relative to the influence of height above the water table varies between watersheds. Some species are found primarily in locations of high stream power, while others are limited to portions of the valley bottom that experience only low stream power.

STORAGE OF MINING-RELATED ZINC IN FLOODPLAIN SEDIMENTS, BLUE RIVER, WISCONSIN

This study investigates spatial patterns of sediment-associated zinc (Zn) storage in floodplain deposits and the potential reintroduction of these metal-contaminated sediments to the active channel by bank erosion. We estimate patterns of Zn mass storage by combining longitudinal trends in Zn concentrations with measurements of sediment mass storage in overbank and point-bar deposits. Overbank deposits are the largest contaminant sink, storing five times more Zn than the point-bar deposits. While Zn concentrations decrease downstream because of dilution effects, the total mass of Zn stored in floodplains is greatest in both the upstream reaches and in the wider lower valleys where low channel gradients promote rapid sedimentation. Zn storage is low in middle reaches where steep, narrow valleys with high stream power favor sediment transport over deposition. Overall, more than half of the Zn released by mining remains stored in floodplain deposits within the watershed. The remobilization of Zn from storage is more likely in the upstream and mid-basin reaches where high stream power increases rates of lateral channel migration. Channels in the lower valley lack the stream power to migrate laterally and remobilize the large Zn mass stored in overbank sediments. [Key words: floodplains, Zn contamination, mining, stream power, Wisconsin.]

Nonlinear Downstream Changes in Stream Power on Wisconsin's Blue River

Spatial characteristics of stream power are examined along four stream reaches through the 208-km2 Blue River watershed in southwestern Wisconsin. Although prevailing theoretical arguments often use spatially aggregated data and power functions derived from hydraulic geometry to suggest that stream power increases linearly downstream, analysis of stream-power trends along the Blue River's spatially connected stream pathways in southern Wisconsin shows that these pathways exhibit varying degrees of nonlinearity in the downstream distribution of stream power. In three of the four stream pathways, cross-sectional stream power and mean stream power reach maximum values at drainage areas between 10 and 100 km2 before decreasing rapidly downstream. Geomorphic and lithologic controls, which produce unusually steep channel slopes in mid-basin locations, are primarily responsible for the nonlinear downstream trends in stream power. Wide valley bottoms and gentle channel gradients occur in erodible lithologies, while resistant lithologies are associated with narrow valley bottoms and steep channel gradients. Spatial variations in drainage network evolution and past stream flow and sediment conditions also influence modern channel gradients. Downstream variations in cross-sectional stream power along interconnected pathways within the drainage network influence the quantity of post-settlement overbank sediments stored on floodplains. Greater sediment storage occurs in the headwaters and lower valleys where stream power is low, whereas little sediment is stored in mid-basin reaches with high stream power.

Geomorphological adjustment of a newly engineered upland sinuous gravel-bed river diversion: Evan Water, Scotland

Following construction of a sinuous diversion on an upland gravel-bed river two flood events within three days of each other, with estimated recurrence intervals of between 1·5 and 2·0 years, caused marked geomorphological adjustment to the channel. The floods resulted in bank erosion, point bar formation, scour on the outside of meander bends and particle sorting, but the overall stability of the river diversion, which is to flow adjacent to a new motorway embankment was not affected. Moreover, the channel adjustment brought about beneficial changes in terms of increasing morphological diversity: probably enhancing fish habitat, although spawning gravels placed at the head of riffles in the river diversion were scoured. The extent and nature of the geomorphological changes relate principally to the high stream power and bedload transport capacity of the river. Geomorphological principles, critical stream powers and bedload transport rates can account for the size and redistribution of bed material transport. Other factors accounting for the adjustment include limited vegetation colonization of geotextile-covered river banks and construction of overly narrow and high channel constrictions to promote flow convergence and acceleration at the head of downstream pools. The case study provides lessons for future construction of river diversions on high energy gravel-bed rivers. © 1997 John Wiley & Sons, Ltd.

Spatial patterns of historical overbank sedimentation and floodplain evolution, Blue river, Wisconsin

Increased soil erosion and overbank flooding associated with land use changes following European settlement have buried pre-settlement floodplain surfaces in many Driftless Area watersheds with as much as several meters of alluvium. This paper examines the processes influencing spatial variations in the storage of post-settlement vertical accretion alluvium and the impact on floodplain evolution in the Blue River watershed, Wisconsin. The cross-sectional area of historical vertical accretion sediments stored on floodplains was measured at 41 cross-valley transects in the 208 km 2 watershed. The storage of historical overbank sediments is influenced by valley width, stream power, and the development of meander belts. Post-settlement alluvial storage increases with valley width and decreases with cross-sectional stream power. In a multivariate regression model, valley width and cross-sectional stream power explain 67% of the variance in the amount of post-settlement alluvium. The development of meander belts, enlarged channels confined between alluvial terraces, by accelerated lateral channel migration influences historical floodplain storage by decreasing the influence of valley width and modifying stream power characteristics for flows, greater than bankfull. Meander belts are located in headwater and mid-basin valley reaches where high stream power produces rapid lateral channel migration, whereas low gradient channels in downstream valleys lack the power necessary to migrate laterally and develop meander belts. Meander belts influence the spatial pattern of historical sedimentation by: (1) containing large magnitude floods and preventing overbank flow, (2) increasing sediment-transport capacities, (3) initiating the erosion of previously deposited historical sediments, (4) reducing the attenuation of flood peaks, and (5) increasing the amount of sediment routed downstream to wide, low gradient valley bottoms lacking meander belts. Continued high rates of lateral channel migration will eventually be damped as widening of the meander belt decreases flow depths and stream power.

Among-Site Variation in Riparian Vegetation of the Southern California Transverse Ranges

-Data from 37 sites in the Transverse Ranges in southern California were used to assess the influence of a variety of environmental variables on among-site variation in riparian vegetation. Ordination and regression analyses indicated that elevation, unit stream power, fire history and valley width all have significant impacts on the composition of this vegetation. TWINSPAN classification generated four species groups: a Baccharis glutinosa/Salix laevigata group that appears related to low elevations, high stream power and recent fire; a Populusfremontii/Alnus rhombifolia group associated with high stream power, but longer time since burning and higher elevations; a Salix lasiolepis/Rosa californica group typically found at high elevation sites with low stream power; and a Platanus racemosa/Adenostoma fasciculatum group that did not show a clear environmental preference.