Stream Incision Research Articles

Geology and stratigraphy of the south-eastern Lake Edward basin (Petroleum Exploration Area 4B), Albertine Rift Valley, Uganda

The Lake Edward basin lies within the Albertine Rift Valley of Uganda and the Democratic Republic of Congo which forms the northern end of the western arm of the East African Rift System. It is a frontier petroleum prospective area, which, at the outset of this study, had no exploration wells drilled within it or any deep reflection seismic surveys. There have been some previous studies in the basin, but none produced a geological map subdividing the onshore rift-fill sediments or established a workable stratigraphic framework for them. Between 2007 and 2010, Dominion Uganda Ltd., in collaboration with Trinity College Dublin and the Petroleum Exploration and Production Department of the Ministry of Energy, Uganda, undertook a geological mapping survey of the south-eastern onshore part of the basin, known as petroleum ‘Exploration Area 4B’ (EA4B). Five rift sediment formations were identified and mapped across the area to produce a new geological map of EA4B. Palynological analyses suggest that all exposed rift sediments are (Late to Mid) Pleistocene–Holocene. EA4B is dominated by a north-east to south-west trending fault zone which underwent significant extension within the last 130,000 years to produce a trough, or sub-basin, to the south-east against the rift margin. This trough subsequently filled, initially with ponded swamp clays, followed by coarse fluvial and alluvial clastics. There is field evidence for minor inversion and ‘pop-up’ structures along some footwall crests, suggesting that the neotectonic phase is compressional or transpressional, and this has caused stream rejuvenation and incision.

Numerical simulation of landscape evolution and water run-off on digital elevation models obtained from Pleiades

This paper illustrates how the main physical laws proposed in landscape evolution models (LEMs) can be modeled by a system of three partial differential equations governing water run-off, stream incision, hill slope evolution and sedimentation. Several numerical experiments on high resolution digital elevation models (DEMs) obtained from image stereo pairs of the satellite Pleiades illustrate its potential to simulate the fine structure of the river network, and to understand the landscape morphology and its causes.

Development of Gulley Ali Beg Gorge in Rawandooz Area, Northern Iraq

The northern and northeastern parts of the Iraqi exhibit different types of gorges. Some of them are few kilometers in length and few hundred meters in height. The most significant gorge is the Gulley Ali Bed gorge in Rawandooz area; its length is about 12.1 Km, with maximum and minimum widths of 100 to 155 m, respectively. The water in the gorge flows in two opposite directions, from the north and south through two streams; they meet in the middle of the gorge, and then change the streams direction towards west; for few kilometers before it merges in the Greater Zab River. The gorge runs within very hard thickly bedded to massive carbonate rocks, which belong to Sarmord, Qamchuqa and Bekhme formations (Cretaceous in age). To the right of the southern entrance; stands Korak anticline, whereas to the left occurs Peris anticline. To the right of the northern entrance; stands Handreen anticline, whereas to the left is Bradost anticline; represented by Balikian Mountain. The gorge is developed by continuous lateral and vertical growth of the four mentioned anticlines, with continuous incision of both streams, Rawandooz from the north and Khlaifan from the south. Both shifted their courses, continuously as indicated from the presence of many wind gaps in the aforementioned anticlines.

Groundwater lowering and stream incision rates in the Central Appalachian Mountains of West Virginia, USA

Surface channel incision rates are of broad geomorphological interest because they set the boundary conditions for landscape change by affecting changes in local relief and hillslope angles. We report groundwater table lowering rates associated with subsurface Buckeye Creek and the surface channel of Spring Creek in southeastern West Virginia, USA. The mountainous watersheds have drainage areas of 14 km 2 and 171 km 2 , respectively. The lowering rates are derived from U/Th-dating of stalagmites and the paleomagnetostratigraphy of clastic sediments in Buckeye Creek Cave. The oldest stalagmites have a minimum age of 0.54 Ma and we use a minimum age of 0.778 Ma for clastic cave sediments deposited during a period of reversed magnetic polarity. The water table at Buckeye Creek has lowered at a rate of ≤40 m Ma -1 . Based on the relative elevations of Buckeye and Spring creeks, the water table at Spring Creek has lowered at a rate of ≤47 m Ma -1 . These values are consistent with previously published rates obtained from caves in the region, although those rates were reported as surface channel incision rates, based on the assumption local groundwaters drained to the surface channel of interest. However, the rates we report are almost certainly not simple bedrock incision rates because of karst processes acting within the cave and surrounding, well-developed fluviokarst (e.g., stream capture). Caveats aside, incision rates of ≤47 m Ma -1 now appear typical of landscapes of the Appalachian Mountains and Plateau.

Characterising the spatial distribution, frequency and geomorphic controls on landslide occurrence, Molise, Italy

An 815-km2 area in Molise, central Italy, was used as a natural laboratory to characterise the lithological, topographic, and fluvial controls on the spatial distribution and frequency of active landslides in temperate climates. The study area is part of the central Apennines fold-and-thrust belt and is characterised by the presence of a tectonic mélange of scaly clays, known as Argille Varicolori, as well as more competent limestone and sandstone units. The area has been subjected to a cumulative stream incision of up to 400m since the mid-Pleistocene. The landslide inventory was developed by combining satellite image interpretation and existing data sets, ground-truthed in the field. Statistical analysis of the inventory data demonstrates that the coupling between landslide occurrence in the area and lithology is strong, with landslides occurring 3–10 times more frequently where the mechanically weak Argille Varicolori (scaly clays) outcrops compared to other lithologies. Analysis of the relationship between landslide occurrence and hillslope gradients shows that 50% of all landslides occur on slopes between 10 and 15°, despite this interval representing only 30% of hillslope gradients in the study area. These results demonstrate the major control that the properties of the regionally significant Argille Varicolori have on landslide occurrence in the area, directly and through their control on hillslope gradients. Additionally, our results show that more than 50% of the landslides occur at a distance of <260m from the nearest stream, a value which is 4 times smaller than the median distance (~1km) between the interfluves and the valley floors within the study area. These data therefore demonstrate the important role that ongoing fluvial incision plays in governing the locus and frequency of landslides in an area in which the streams incise at rates of 0.4 to 0.7mm/y. Finally, an analysis of the landslide size–frequency distribution revealed that the power-law exponent on the area term is 1.56±0.2. This is toward the lower range of values published in the literature and suggests that in this part of the central Apennines active landslide frequency declines more slowly with size than is often modelled. More widely, this study provides new insights into the coupling between fluvial erosion and landsliding in an area of active tectonics and illustrates the fundamental role of lithologies such has the Argille Varicolori in determining landslide susceptibility.

Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

40Ar/39Ar laser incremental heating analyses of supergene K-Mn oxides from weathering profiles at the Baye Mn deposit, southern Yunnan Plateau, SW China, were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution. Weathering profiles in the Baye Mn mine are dominated by 20–30m thick saprolites, which are locally covered by unconsolidated alluvial deposits. We analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations. In most cases, different grains from the same hand sample and different samples from the same site have reproducible 40Ar/39Ar plateau ages. The plateau ages of individual grains are also consistent with the respective isochron, integrated, and ideogram ages. This age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles.Sixty-four grains from 32 hand samples collected from weathering profiles at four sites (A to D) yield well-defined plateau or pseudoplateau ages ranging from 2.98±0.07 to <0.16Ma (2σ). The age results show that Mn oxides become progressively younger towards lower elevations: samples collected from the highest elevation site (∼1855m a.s.l.) have the oldest ages at 2.98±0.07 to 2.83±0.13Ma; samples from intermediate elevations (1821 to 1815m a.s.l.) yield ages ranging from 2.91±0.10 to 2.08±0.11Ma; and those from the lowest sites (1768 to 1753m a.s.l.) are younger than 1.25±0.08Ma. The age versus elevation relationship indicates fast downward propagation of weathering front since the late Pliocene, and permits estimating the weathering rates at 3.3±3.8, 6.6±1.2, and 11.1±1.9mMyr−1 during the time periods of 2.98–2.83, 2.91–2.08, and 1.25 to <0.16Ma, respectively. The average river incision rates estimated from the surface exposed ages range from 235±10 to 416±19mMyr−1 during the past 3 million years. The incision rate calculated from minimum exposed ages at sites B–D is 33±2mMyr−1 for the time period of 2.91–1.25Ma, which is consistent with the slow incision of streams on the ridges over the Yunnan Plateau since the early Pleistocene.Three Mn oxide pebbles contained in the alluvial deposits overlying the saprolite-dominated weathering profiles at the two highest sites provided additional information on the timing of weathering. Four grains from two pebbles yield plateau ages of 6.32±0.19 to 5.27±0.10Ma, whereas the other two grains from the third pebble indicate the minimum formation ages of 8.2±0.4 and 9.3±0.3Ma. These ages confirm the existence of older weathering profiles, now dismantled, in the region. Manganese oxide 40Ar/39Ar ages of the Baye deposit, when combined with results from other localities, indicate that lateritic weathering and supergene Mn enrichment and, by inference, warm and humid climates conducive to intense weathering have prevailed over the Plateau since the middle to late Miocene. The climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records, confirming that supergene Mn oxides can be used as a useful proxy for past climate. Age clusters of Mn oxides at 2.9–2.4, 1.2–0.8, and 0.6–0.4Ma are broadly coincident with and thus likely reflect intensification events of the Indian Summer Monsoon that brings moisture and abundant precipitation to the Yunnan Plateau. These clusters also coincide with the periods of significant surface uplift in the Yunnan Plateau, demonstrating a causal link between topographic evolution, plateau uplift, and intensification of the monsoonal climate.

Inner gorge–slot canyon system produced by repeated stream incision (eastern Alps): Significance for development of bedrock canyons

Many inner bedrock gorges of the Alps show abrupt downstream changes in gorge width, as well as channel type and gradient, as a result of epigenetic incision of slot canyons. Many slot canyons also are associated with older gorge reaches filled with Quaternary deposits. The age of slot canyons and inner bedrock gorges, however, commonly is difficult to constrain. For the inner-bedrock gorge system of the Steinberger Ache catchment (eastern Alps), active slot canyons as well as older, abandoned gorge reaches filled with upper Würmian proglacial deposits record three phases of gorge development and slot–canyon incision. A 234U/230Th age of cement of 29.7±1.8ka in fluvial conglomerates onlapping the flank of an inner gorge fits with late Würmian valley-bottom aggradation shortly before pleniglacial conditions; in addition, the age indicates that at least the corresponding canyon reach must be older. During advance of ice streams in the buildup of the Last Glacial Maximum (LGM), the catchment was blocked, and a proglacial lake formed. Bedrock gorges submerged in that lake were filled with fluviolacustrine deposits. During the LGM, the entire catchment was overridden by ice.During post-glacial reincision, streams largely found again their preexisting inner bedrock canyons. In some areas, however, the former stream course was ‘missed’, and a slot canyon formed. The distribution of Pleistocene deposits, the patterns of canyon incision, and the mentioned U/Th cementation age, however, together record a further discrete phase of base-level rise and stream incision well before the LGM. The present course of Steinberger Ache and its tributaries is a patchwork of (1) slot canyons incised during post-glacial incision; (2) vestiges of slot canyons cut upon an earlier (middle to late Würmian?) cycle of base-level rise and fall; (3) reactivated reaches up to ~200m in width of inner bedrock gorge that are watershed at present, and more than at least ~30ka in age; and (4) abandoned, sediment-filled reaches of bedrock canyons that also must be older than 30ka and that are exposed alongside the active streams. ‘Multi-cyclic’ bedrock canyon systems composed of reaches of markedly different ages may be common in mountain ranges subject to glaciations and/or mass wasting.

Watershed‐scale modeling of streamflow change in incised montane meadows

AbstractLand use practices have caused stream channel incision and water table decline in many montane meadows of the Western United States. Incision changes the magnitude and timing of streamflow in water supply source watersheds, a concern to resource managers and downstream water users. The hydrology of montane meadows under natural and incised conditions was investigated using watershed simulation for a range of hydrologic conditions. The results illustrate the interdependence between: watershed and meadow hydrology; bedrock and meadow aquifers; and surface and groundwater flow through the meadow for the modeled scenarios. During the wet season, stream incision resulted in less overland flow and interflow and more meadow recharge causing a net decrease in streamflow and increase in groundwater storage relative to natural meadow conditions. During the dry season, incision resulted in less meadow evapotranspiration and more groundwater discharge to the stream causing a net increase in streamflow and a decrease in groundwater storage relative to natural meadow conditions. In general, for a given meadow setting, the magnitude of change in summer streamflow and long‐term change in watershed groundwater storage due to incision will depend on the combined effect of: reduced evapotranspiration in the eroded meadow; induced groundwater recharge; replenishment of dry season groundwater storage depletion in meadow and bedrock aquifers by precipitation during wet years; and groundwater storage depletion that is not replenished by precipitation during wet years.

Hydrogeology of Pumice-Hosted Fens in the Winema-Fremont National Forest, Oregon, USA

Subaerial fallout from the Holocene eruption of Mount Mazama in the Oregon Cascade Range was deposited upon relatively low permeability volcanic and volcaniclastic bedrock and regolith. In the Walker Rim study area, erosion by ephemeral streams shortly after the eruption disrupted the lateral continuity of the 270 to 300 cm-thick pumice deposit. Co-evolution of the surface- and ground-water systems in a low-relief, low-slope landscape allowed diffuse groundwater discharge from the banks of the evolving stream system. Accumulation of organic material from groundwater dependent ecosystems at these sites of discharge allowed peat deposits to form on gently sloping erosion surfaces cut into the pumice deposit. Following early stream incision, fine-grained, silt-rich deposits accumulated in valleys and contributed permeability barriers to the lateral migration of water in the pumice aquifer. Fens discharge from the pumice aquifer through gently sloping surfaces patterned after the slope of the erosion surface cut into the pumice deposit and overlain by approximately 1 m of peat on the sloping surface and alluvium or iron-cemented pumice overlain by alluvium at the toe of the slope. The predominant source of groundwater is snowmelt which infiltrates the pumice deposit during the freshet. However, shallow groundwater flow also takes place along permeable pathways in bedrock units. Locally, low volume discharge takes place along faults. The snowmelt-dependent hydrologic system that supports the fens of the Walker Rim study area occurs at elevations primarily above 1585 m.

Morphometric Analysis of Drainage Basins in the Western Arabian Peninsula Using Multivariate Statistics

The uplift of the Arabian Shield and the opening of the Red Sea led to the development of steep drainage systems in the Western Arabian Peninsula. Although the Peninsula has been studied from a geological perspective, in relation to oil production, plate tectonics and eolian systems such as sand dunes, the steep mountainous drainage basins have received much less attention. This paper aims to assess the characteristics and development of 36 drainage basins in the Western Arabian Peninsula, using a digital elevation model (DEM), principal component analysis (PCA), and hierarchical cluster analysis (CA). Three major principal components (PC1 to PC3) are found to explain 73% of total variance. CA divided the basins into two or four groups. The division by CA strongly reflects PC1, showing that the two analyses give comparable results. PC1 strongly reflects basin dimensions and drainage texture, and their positive correlations indicate the significant effect of basin relief and slope on mass wasting and limited stream incision in small basins under an arid climate. PC2 mainly reflects the effect of bedrock geology, suggesting that volcanic rocks tend to produce more elongated and less eroded immature basins than crystalline rocks do. PC3 mainly reflects the basin relief and slope and the length of each stream segment, which may also reflect the effect of mass wasting on stream development.

Deep-seated gravitational slope deformations in western Sicily: Controlling factors, triggering mechanisms, and morphoevolutionary models

A study of deep-seated gravitational slope deformation (DSGSD) phenomena affecting areas of various geological and geomorphological settings in western Sicily is described. Western Sicily is underlain by a thin-skinned imbricate wedge of Meso–Cenozoic carbonate and siliciclastic rocks that formed by the stacking of several thrust nappes over the Iblean foreland. Locally, the original thrust sheets are folded and cut by high-angle faults. Large areas of western Sicily now display high relief energy due to Plio-Pleistocene block-faulting and uplifting, and the Quaternary morphogenetic phases are characterised by incision, thereby triggering widespread DSGSDs. To identify controlling factors and triggering causes and to develop reliable morphoevolutionary models for the DSGSDs of western Sicily, a geomorphological study based on field surveys and aerial-photography interpretations was performed. Previous geomorphological data relating to well-known examples of DSGSDs were reconsidered, leading to remarkable revisions of the interpretative models in certain cases. New data were subsequently collected, enabling recognition of additional DSGSD phenomena. The whole body of data involves a total of 27 DSGSDs affecting areas in two specific geological settings: (1) areas with flat thrust surfaces, where differential settlements, back-tilting, lateral spreads in competent rocks overlying marls and clays, large topples, and/or block-type slope movements may develop; and (2) areas where deep-rooted carbonate units come into lateral contact with clayey–marly units along high-angle faults, where lateral spreads in brittle homogeneous rocks, sinking, and/or rock flows may occur. These DSGSD phenomena are associated with different evolutionary stages, allowing a morphoevolutionary model to be defined for the two geological conditions. For these two different morphoevolutionary models, the following structural features play an important role in the development of DSGSDs: (1) where carbonate bodies overlie clayey–marly rocks, triggering of the DSGSDs traces back to deformation of a ductile substratum that follows the exhumation of the flat thrust planes and the underlying clayey–marly rocks due to block-faulting and/or stream incision; and (2) where homogeneous carbonate rocks hundreds of metres thick crop out, the DSGSDs are triggered by very high relief energy and tensile stress that follow the combined actions of Quaternary block-faulting and stream deepening associated with differential erosion.

Climatic imprint on landscape morphology in the western escarpment of the Andes

Because of competing forces and variations through time, the relative importance of geomorphic processes responsible for the long-term topographic evolution of a mountain range is not always obvious. Here we perform a space-for-time substitution with the western escarpment of the Andes between 10 and 20°S to identify the mechanisms of plateau destruction over geological timescales. We use this setting to propose that variations in the precipitation rate play a primary role in setting hillslope relief in uplifted mountainous landscapes. We find that in dry climates local topographic relief grows with increasing precipitation, independent of the underlying lithology and given an overall uniform rock uplift history. We proceed by differentiating Andean landscapes with generally low precipitation rates (80–500mma−1, Peruvian Andes 10–20°S) where local relief correlates positively with precipitation, from those with higher precipitation rates (400–1400mma−1, Chilean Andes 35–40°S) where increases in precipitation lead to topographic decay. We suggest that these trends result from dominant bottom-up control (channel incision is faster than hillslope response) giving way to an increasing top-down control (hillslope lowering is faster than channel incision). With low precipitation, relief growth is controlled by stream incision and knickzone retreat into a largely undissected plateau. With higher precipitation rates, relief is set by the steepness of graded streams and the rates of sediment production and transport on hillslopes.Trends of topography can also be interpreted in temporal terms in which the higher precipitation results in shorter response times, such that the Peruvian Andes between 10 and 20°S are still responding to Miocene uplift, while in the Chilean Andes between 35 and 40°S, these knickzones have already propagated through the entire fluvial network. We anticipate that such changes also operate during the formation and destruction of other mountainous plateau landscapes.

Structural control on the presence and character of calas: Observations from Balearic Islands limestone rock coast macroforms

Calas are characteristic embayed rocky coastline landforms related to steep-sided drowned valleys that were deeply incised during low sea-level stages into carbonate plateaus. This paper studies the factors that control the presence and character of calas in three different islands of the Balearic Archipelago (Mallorca, Menorca and Formentera) by means of Digital Elevation Model (DEM) reconstructions, embayment and catchment morphometric analysis and through the compilation of fault and joint orientations. All islands exhibit emerged carbonate platforms (Upper Miocene in age) characterised by horizontal or very gently-dipping bedding. Two distinct patterns observed at all study sites are the absence or poor development of calas when carbonate plateaus exhibit strong coast-parallel structures and a greater variability of cala embayment size according to stream catchment size, lithology or their catchment distance from the coastline. Furthermore, the amount of stream incision, and consequently the characteristics of a cala at its mouth, depends on the proportion of impermeable rocks exposed in each drainage basin and on the density and spacing of fractures. Karst collapse, dolines and valley infilling associated with Quaternary sea-level oscillations are also important in determining the shape and size of calas.

Paleochannels, stream incision, erosion, topographic evolution, and alternative explanations of paleoaltimetry, Sierra Nevada, California

Geologic relationships in the Sierra Nevada, California, show negligible stream incision between Eocene and Late Miocene–Pliocene time. Stream incision of up to ∼1 km began at (from south to north) ca. 20 Ma in the Kern to Kings River drainages, between 6 and 10 Ma in the San Joaquin River drainage, 3.6–4 Ma in the Stanislaus and Mokelumne River drainages, and ca. 3 Ma in the American and Feather River drainages. These differences in incision timing greatly exceed the time of knickpoint retreat, based on the example of the North Fork Feather River, where the knickpoint may have retreated over 100 km in less than 300 k.y. based on ages of interfluve-capping andesites and an inset basalt flow. The knickpoint in the Stanislaus River may have retreated over 50 km in less than 400 k.y. based on somewhat looser constraints. Eocene paleochannels show lowest gradients parallel to the range axis, steepest ones perpendicular, and reaches with significant “uphill” gradients that rise in the paleo-downstream direction. Modern Sierran rivers lack this relationship. The azimuth-gradient relationships of paleochannels, especially the uphill gradients, require late Cenozoic tilting and uplift. Incision began in spite of decreasing discharge and increasing sediment load and must have resulted from steepening associated with tilting and uplift. Stable-isotope paleoaltimetry apparently records a profile similar to that of the modern range and areas east of it, in spite of significant vertical deformation that postdates the age of the sampled deposits, suggesting fairly recent reequilibration, in contrast to the published interpretations of closed-system behavior since the Oligocene or Eocene. Such apparent open-system behavior agrees with studies showing progressive hydration of volcanic glass and the correspondence between weathering and erosion rates. Northward-younging initiation of late Cenozoic uplift and stream incision suggests a relationship with triple-junction migration, possibly associated with slab window development, with a second uplift pulse related to delamination and limited to the southern Sierra (San Joaquin River drainage and southward). Basement features may have significantly influenced along- and across-strike differences in Cenozoic tectonics and geomorphic response.

Formation of phreatic caves in an eogenetic karst aquifer by CO2 enrichment at lower water tables and subsequent flooding by sea level rise

ABSTRACTFormation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater. Extensive phreatic caves also occur, however, in eogenetic karst aquifers where fresh and saltwater do not mix, for example in the upper Floridan aquifer. These caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs. Alternatively, these caves have been hypothesized to have formed at lower water tables during sea level low‐stands. These hypotheses have not previously been tested against one another. Analyzing morphological data and water chemistry from caves in the Suwannee River Basin in north‐central Florida and water chemistry from wells in the central Florida carbonate platform indicates that phreatic caves within the Suwannee River Basin most likely formed at lower water tables during lower sea levels. Consideration of the hydrological and geochemical constraints posed by the upper Floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose CO2 gas into the groundwater. Sea level rise and a wetter climate during the mid‐Holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table. When rising water tables reached the land surface, surface streams formed. Incision of surface streams breached the pre‐existing caves to form modern springs, which provide access to the phreatic caves. Phreatic caves in the Suwannee River Basin are thus relict and have no causal relationship with modern surficial drainage systems. Neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers. Dissolution at water tables, potentially driven by vadose CO2 gas, offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Effects of active folding and reverse faulting on stream channel evolution, Santa Barbara Fold Belt, California

The Santa Barbara Fold Belt (SBFB) is an area of active crustal shortening comprising younger east–west and older southeast-northwest oriented reverse faults and folds. Changes in channel position and stream incision are the result of active fold growth and uplift. Rates of incision range from 0.4±0.1m/ka to 1.2±0.04m/ka. Lateral stream diversion from fold and fault growth varies from 6.7 to 0.4km. Minimum uplift of western Mission Ridge anticline is 0.8±0.1m/ka and minimum uplift on the Mesa fault is 0.3–0.4m/ka. The hypothesis that east-to-west folds are younger is suggested by a cross-cutting relationship and is tested using geomorphic indices of active tectonics including: valley width to height ratios (Vf), mountain front sinuosity (Smf) and drainage densities (Dd). East-to-west trending structures have mean values of Vf, Smf, and Dd of 2.7, 1.2, and 2.2km/km2, respectively, while southeast-to-northwest trending structures have mean values of 5.0, 1.5, and 3.7km/km2, respectively. We present a new geomorphic tool, the stream diversion index, which uses diversion timing and distance to assess relative tectonic activity; these values range from 6 to 260m/ka. All of these values support the hypothesis that the east-to-west folds are younger and more active. Clockwise rotation of the Western Transverse Ranges is probably the process that produced the two sets of structures. With rotation the older set that started out east-to-west eventually became northwest-to-southeast, and more difficult to maintain with the north-to-south contractional (shortening) associated with the Big Bend of the San Andreas fault 80km to the north. The second set of younger structures formed with a more favorable (east-to-west) orientation to the shortening.

Influence of CO2 dynamics on the longitudinal variation of incision rates in soluble bedrock channels: Feedback mechanisms

We use measurements of dissolution rates of limestone tablets placed along a cave stream to estimate rates of modern incision. Dissolution rates within the stream display a systematic decrease with downstream distance. We discuss a variety of mechanisms that could be responsible for the longitudinal decrease in dissolution rates and develop simple mathematical models for each. The dissolutional length scales that arise from each model allow a first-order estimate of the plausibility of each mechanism and motivate further field studies to test each possibility. Water chemistry and other field data suggest that a decrease in the concentration of CO2 along the cave stream is responsible for the observed decrease in dissolution rates. We propose two potential mechanisms that could trigger this reduction in dissolved CO2 and discuss the plausibility of each mechanism in light of the field data collected. Either of these mechanisms introduces a feedback loop whereby the stream profile of a channel in soluble bedrock indirectly influences CO2 concentrations in the water, via either microbial or hydraulic processes. The CO2 concentration in turn effects the incision rates and therefore the future stream profile. This study illustrates the importance of CO2 dynamics in determining incision rates in a soluble channel and points to further modeling and field work that are needed in order to enable the development of realistic stream incision models in soluble strata.

Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA)

In high-latitude catchments where permafrost is present, runoff dynamics are complicated by seasonal active-layer thaw, which may cause a change in the dominant flowpaths as water increasingly contacts mineral soils of low hydraulic conductivity. A 2-year study, conducted in an upland catchment in Alaska (USA) underlain by frozen, well-sorted eolian silt, examined changes in infiltration and runoff with thaw. It was hypothesized that rapid runoff would be maintained by flow through shallow soils during the early summer and deeper preferential flow later in the summer. Seasonal changes in soil moisture, infiltration, and runoff magnitude, location, and chemistry suggest that transport is rapid, even when soils are thawed to their maximum extent. Between June and September, a shift occurred in the location of runoff, consistent with subsurface preferential flow in steep and wet areas. Uranium isotopes suggest that late summer runoff erodes permafrost, indicating that substantial rapid flow may occur along the frozen boundary. Together, throughflow and deep preferential flow may limit upland boreal catchment water and solute storage, and subsequently biogeochemical cycling on seasonal to annual timescales. Deep preferential flow may be important for stream incision, network drainage development, and the release of ancient carbon to ecosystems.

Consistency of groundwater flow patterns in mountainous topography: Implications for valley bottom water replenishment and for defining groundwater flow boundaries

Topographic influences on groundwater flow processes that contribute to baseflow and mountain block recharge (MBR) are conceptually investigated using three‐dimensional numerical models of saturated groundwater flow. Model domains for conceptual and real topographies are developed as “mountain groundwatershed units” (MGUs) to represent regional‐scale watershed systems. Results indicate regularity in groundwater flow patterns that reflect consistency of prominent topographic features, providing a basis for conceptualizing three‐dimensional groundwater flow. Baseflow is generated mainly from recharge within the watershed area. MBR is produced primarily from recharge that is focused across triangular facets near the mountain front (∼73%–97% of total MBR), with additional contributions originating within the watershed (up to ∼27% of MBR). MBR contributions originating from recharge near the highest‐elevation watershed boundaries are minimal but are greater for topography with less stream incision. With orographic influences, more MBR originates within the watershed. MBR rates are relatively consistent between models because of similarities in mountain front topography, while baseflow is variable. Gains and losses to systems via cross‐watershed groundwater flux, generated because of topographic differences between adjacent watersheds, cause baseflow to vary by up to ∼10% but do not significantly influence MBR. In data‐sparse regions such as mountains, a basic numerical modeling approach, using the MGU concept with topography data and mapped watershed boundaries, can be used to develop site‐specific conceptual models to constrain water budgets, to delineate recharge areas, and to guide further investigation and data collection.

A numerical model for meltwater channel evolution in glaciers

Abstract. Meltwater channels form an integral part of the hydrological system of a glacier. Better understanding of how meltwater channels develop and evolve is required to fully comprehend supraglacial and englacial meltwater drainage. Incision of supraglacial stream channels and subsequent roof closure by ice deformation has been proposed in recent literature as a possible englacial conduit formation process. Field evidence for supraglacial stream incision has been found in Svalbard and Nepal. In Iceland, where volcanic activity provides meltwater with temperatures above 0 °C, rapid enlargement of supraglacial channels has been observed. Supraglacial channels provide meltwater through englacial passages to the subglacial hydrological systems of big ice sheets, which in turn affects ice sheet motion and their contribution to eustatic sea level change. By coupling, for the first time, a numerical ice dynamic model to a hydraulic model which includes heat transfer, we investigate the evolution of meltwater channels and their incision behaviour. We present results for different, constant meltwater fluxes, different channel slopes, different meltwater temperatures, different melt rate distributions in the channel as well as temporal variations in meltwater flux. The key parameters governing incision rate and depth are channel slope, meltwater temperature loss to the ice and meltwater flux. Channel width and geometry are controlled by melt rate distribution along the channel wall. Calculated Nusselt numbers suggest that turbulent mixing is the main heat transfer mechanism in the meltwater channels studied.