Cutbank Erosion Research Articles

Permafrost Formation in a Meandering River Floodplain

AbstractPermafrost influences 25% of land in the Northern Hemisphere, where it stabilizes the ground beneath communities and infrastructure and sequesters carbon. However, the coevolution of permafrost, river dynamics, and vegetation in Arctic environments remains poorly understood. As rivers meander, they erode the floodplain at cutbanks and build new land through bar deposition, creating sequences of landforms with distinct formation ages. Here we mapped these sequences along the Koyukuk River floodplain, Alaska, analyzing permafrost occurrence, and landform and vegetation types. We used radiocarbon and optically stimulated luminescence (OSL) dating to develop a floodplain age map. Deposit ages ranged from modern to 10 ka, with more younger deposits near the modern channel. Permafrost rapidly reached 50% areal extent in all deposits older than 200 years then gradually increased up to ∼85% extent for deposits greater than 4 Kyr old. Permafrost extent correlated with increases in black spruce and wetland abundance, as well as increases in permafrost extent within wetland, and shrub and scrub vegetation classes. We developed an inverse model to constrain permafrost formation rate as a function of air temperature. Permafrost extent initially increased by ∼25% per century, in pace with vegetation succession, before decelerating to <10% per millennia as insulating overbank mud and moss slowly accumulated. Modern permafrost extent on the Koyukuk floodplain therefore reflects a dynamic balance between widespread, time‐varying permafrost formation and rapid, localized degradation due to cutbank erosion that might trigger a rapid loss of permafrost with climatic warming.

Four Years of Meander‐Bend Evolution Captured by Drone‐Based Lidar Reveals Lack of Width Maintenance on the White River, Indiana, USA

AbstractMeandering rivers experience fluctuations in width whenever riverbanks migrate in different directions or at different rates, which can be observed after individual floods. However, meandering rivers maintain approximately constant widths over decadal timescales. This implies some timescale below which width fluctuates as banks migrate independently, and above which width is maintained by a bank‐coupling process. This coupling is thought to occur either as point bar deposition events induce cutbank erosion (bar‐push), or as cutbank erosion events induce point bar deposition (bank‐pull). This coupling, however, has been challenging to observe in natural rivers due to limited event‐scale field data. We present results from a 4.5‐year campaign with 22 drone‐based lidar surveys of a single point bar and cutbank (∼0.35 km2 in area) on the White River near Worthington, Indiana, USA. The middle point bar experienced net erosion (5,400 m3), but net aggradation (17,100 m3) between 2019 and 2022 when including perennially submerged regions. This aggradation was less than the 35,700 m3 of cutbank erosion over the same period. Combined, we have observed widening (1.58 m/yr bend‐averaged; 3.08 m/yr near apex) over the study period as point bar deposition has not kept up with cutbank erosion. Finally, we suggest that the difference between bar‐push and bank‐pull as width‐maintenance mechanisms may not be resolvable by observing bend widening or narrowing alone without an advancement of current theory, such as determining a long‐term equilibrium width and measuring deviations relative thereto.

Meandering river evolution in an unvegetated permafrost environment

Meandering river sedimentary facies are associated with the rise of terrestrial vegetation in the rock record, but modern examples of highly sinuous, single-threaded streams are also found in many modern non-vegetated environments. Prior workers have speculated that permafrost or ice-cemented sediment could provide the cohesion needed to generate single-threaded, meandering channels, however, the presence of bank vegetation in the overwhelming majority of terrestrial permafrost sites in which meandering rivers are present has made it impossible to disentangle the relative impact of bank cohesion provided by frozen sediments from that provided by roots. Here, we examine topographic change over 13 years at the Onyx River, an unvegetated, permafrost-affected, sand-and-gravel-bottomed river in the McMurdo Dry Valleys (MDV) of Antarctica, supported by ∼40 years of image data. On the basis of lidar change detection and satellite/airborne image analysis, we find that meandering river processes (cut bank erosion, point bar growth, and meander migration) can occur in this ice-cemented, unvegetated permafrost river system. We find that the Onyx River's upper, single-threaded reach is meandering, and that bank cohesion is provided by ice-cemented permafrost. Meander migration rates within the Onyx's meandering reach are extremely close to values predicted based on observations of meander migration rates in other unvegetated rivers, however, the short (∼70 day) duration of Onyx River discharge suggests these migration rates should be considered low estimates, which could be up to ∼5 times greater during the short, summer erosion season. Together, these observations suggest that ice-cemented permafrost is a weak form of bank cohesion that can result in rapid landscape change in response to thermokarstic fluvial erosion, producing dynamic river channels, especially where warm meltwater encounters ice-rich permafrost deposits.

Characterizing the branching architecture of drepanophycalean lycophytes (Lycopsida): an exceptional specimen from the Early Devonian Hunsrück Slate, southwest Germany, and its paleobiological implications

An exceptionally large and well-preserved drepanophycalean lycophyte specimen from the Early Emsian Hunsruck Slate exhibits branching morphology and architecture that suggest previously unrecognized diversity of drepanophycalean lycophytes in the German Early Devonian. The specimen shows long prostrate axes giving rise to both rooting axes and erect leafy, unbranched axes via K-branching. Rhizomatous growth along the substrate promoted rapid vegetative expansion of the plant forming monoclonal patches, potential adaptations to colonizing an unstable deltaic environment. K-branches including dormant buds may have allowed resurrection of the shoot system following burial by sediment or damage to growing shoot apices. Cutbank erosion may have uprooted parts of the plants, which were then transported into the open marine areas and eventually buried in the Hunsruck Slate depositional environment. We hypothesize that remains of Late Silurian/Early Devonian plants (and possibly Prototaxites) may have drifted for many kilometers and were large enough to raft small organisms over considerable distances. Thus, the Late Silurian/Early Devonian may have witnessed the earliest passive oceanic dispersal by rafting of terrestrial invertebrates and therefore marks a crucial time in Earth’s history with respect to this important paleobiogeographical factor.

Sediment and Phosphorous Fluxes Analysis in Aquia Creek, a Sub-watershed of the Chesapeake Bay Basin, VA, USA

Decline in global surface water quality around the world is closely linked to excess sediment and nutrient inputs. This study examined sediment and phosphorus fluxes in Aquia Creek, a fourth-order sub-watershed of the Chesapeake Bay located in Stafford, Virginia. The Revised Universal Soil Loss Equation (RUSLE), sediment delivery ratio (SDR), field sediment traps, bank erosion pins, and LIDAR data, combined with historical aerial images, were used in quantifying rill and inter-rill erosion from the basin, as well as internally generated sediments. Stream water and stream bank soils were analyzed for phosphorus. RUSLE/SDR modeling estimates a basin total sediment flux of 25,247 tons year−1. The greatest calculated soil losses were in deciduous forests and cropland areas, whereas medium and high-intensity developed areas had the least soil loss. Cut-bank erosion ranged from 0.2 to 27.4 cm year−1, and annual bank sediment fluxes were estimated at 1444 Mg, with a corresponding annual mass of phosphorous of 13,760 kg year−1. The highest bank loss estimates were incurred along reaches draining urban areas. Stream water total phosphorous levels ranged from 0.054 μg g−1 during low flows to 134.94 μg g−1 during high discharge periods in autumn and spring. These results show that stormwater management practices in urban areas are limiting runoff water and soil contact, reducing surficial soil loss. However, the runoff acceleration due to expansion of impervious surfaces is progressively increasing the significance of intrinsic sediment and phosphorous sources by exacerbating stream bank erosion and resuspension of internally stored sediments.

Using Time‐Lapse Lidar to Quantify River Bend Evolution on the Meandering Coastal Trinity River, Texas, USA

AbstractTime‐lapse airborne lidar on the coastal Trinity River in Texas, USA, shows profound downstream variability in point bar growth and cut bank erosion resulting from a historically large flood that occurred in 2015. The difference map generated from two surveys covers 55 river bends and captures the transition from quasi‐uniform flow into backwater flow. In the upstream portion of the survey, reach‐averaged deposition is greater than erosion, with high local variability between the two due to the occurrence of bend cutoffs. Point bars shift laterally up to 3 times as much as cut bank retreat in this area. In the backwater zone, cut bank erosion consistently outpaces deposition on point bars and cut bank retreat exceeds the lateral shift of the point bar. Edge‐to‐edge channel width, measured from active scroll bar to outer bank, increases for all bends. This widening is often partially due to point bar widening and movement of the scroll bar away from the channel. The combination of net erosion and limited lateral movement of point bars within the backwater zone implies that during large sustained floods, bank pull instigates channel migration in this area. Farther upstream, net deposition and the high ratio of point bar to cut bank lateral motion imply bar push is more important. Patterns of deposition and erosion within a single bend suggest that channel migration was dominated by downstream bend translation with little bend deformation. Results from this study help to deepen the understanding of coastal river system morphodynamics by connecting changes in the geomorphology to changes in river bend kinematics, hydraulics, and sediment transport.

Quantitative seismic geomorphology of a submarine channel system in SE Brazil (Espírito Santo Basin): Scale comparison with other submarine channel systems

Detailed morphological analyses of a Pleistocene-Holocene submarine channel system in terms of its hierarchical framework, were carried out using a 3D seismic volume from offshore Espírito Santo, SE Brazil. The channel morphology shows marked variations, with five segments (Segments a to e) being identified along its full length. For example, the cross-sectional area of the channel decreases by a factor of 70 from Segment a to Segment c, and is then followed by a nearly four-fold increase from Segment c to Segment d. The significant changes in channel morphology relate to temporal and spatial variations in flow volume within the channel. In the same channel system, the valley reveals three distinct segments (Segments A to C), with similar aspect ratios but marked variations in morphology along the valley distance. Valley morphological changes are chiefly affected by erosional processes. Segment B is characterised by the largest valley-base width, valley width, and cross-sectional area compared to the other two segments. Valley enlargement in Segment B results from relatively high degrees of lateral channel migration and associated cut bank erosion, leading to the widening of the valley, especially the valley base. In Segment C, the valley is characterised by inner bank erosion in the form of shallow-seated mass failures, which only enlarged the upper part of the valley wall. The spatial variations in both channel and valley morphology documented here suggest an important role of local factors (e.g. salt diapirs, tributaries, overbank collapse) in the development of channel systems. Hence, the morphological analyses developed in this work provide an effective tool for studying channels and valleys on continental slopes around the world.

Bank erosion of legacy sediment at the transition from vertical to lateral stream incision

AbstractStreambank erosion is a primary source of suspended sediments in many waterways of the US Atlantic Piedmont. This problem is exacerbated where banks are comprised of fine sediment produced by the intensive land use practices of early European settlers. A stream in this region, Richland Creek incises into banks comprised of three stratigraphic layers associated with historic land use: pre‐European settlement, early European agriculture and development, and water‐powered milldam operation. This study aims to identify the bank processes along a reach of Richland Creek that is eroding towards its pre‐disturbance elevation. The volume of material that has eroded along this stream since the milldam breached was calculated by differencing a reconstructed surface of the pond bed and an aerial lidar digital terrain model (DTM). Immediately downstream from the study reach, the channel is floored by bedrock and immediately upstream the rate of channel erosion approximately doubled along the longitudinal profile of Richland Creek, which indicate that the study reach spans the transition from a channel dominated by vertical incision in the upstream direction to horizontal widening in the downstream direction.The combined hydrometeorological conditions and dominant processes causing reach‐scale cut bank erosion were investigated with analyses of stream stage, precipitation, and streambank volumetric and surfaces change that was measured during nine terrestrial lidar surveys in 2010–2012. The spatial variability of erosion during a simulated precipitation event was examined in a field‐based experiment. Erosion was greatest where mill pond sediment columns detached along vertical desiccation and horizontal seepage cracks. This sediment accumulated on the bank toe throughout the study and was a source of readily‐entrained fine sediment contrary to the upper reaches where depositional accommodation space is more limited. Findings suggest that hotspots of sediment excavation progress upstream, indicating that restoration efforts should focus upon stabilizing banks at these locations. Copyright © 2015 John Wiley & Sons, Ltd.

Modeling planform evolution of a mud‐dominated meandering river: Quinn River, Nevada, USA

ABSTRACTA depth‐averaged linearized meander evolution model was calibrated and tested using the field data collected at the Quinn River in the Black Rock Desert, Nevada. Two approaches used to test the model were: (1) simulating meander evolution and comparing the results with the observed 38 year migration pattern; and (2) fitting the model parameters to present bank asymmetry (the ratio of the maximum bank gradients on opposite sides of the channel). The data required as input were collected in the field during a high flow in May 2011 and from aerial photographs and LiDAR data. Both approaches yielded similar results for the best fit parameter values. The bank asymmetry analysis showed that the bank asymmetry and the velocity perturbation have high correlation at close to zero spatial lag while the maximum correlation between the bank asymmetry and maximum bend curvature is offset by about 25 m. The model sufficiently replicated 38 years of channel migration, with a few locations significantly under‐ or over‐predicted. Inadequacies of the flow model and/or variation in bank properties unaccounted for are most likely the causes for these discrepancies. Flow through the Quinn River was also simulated by a more general 3D model. The downstream pattern of near‐bank shear stresses simulated by the 3D model is nearly identical to those resulting from the linearized flow model. Topographic profiles across interior bends are essentially invariant over a wide range of migration rates, suggesting that the traditional formulation that cut bank erosion processes govern migration rates is appropriate for the Quinn River. Copyright © 2014 John Wiley & Sons, Ltd.

Mediative adjustment of river dynamics: The role of chute channels in tropical sand-bed meandering rivers

This paper examines processes of chute channel formation in four tropical sand-bed meandering rivers; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics-based modelling work. GIS analyses indicate that bend widening may be driven by a variety of mechanisms, including scour and cutbank bench formation at sharply-curving bends, point bar erosion due to cutbank impingement against cohesive terrace material, rapid cutbank erosion at rapidly extending bends, and spontaneous mid-channel bar formation. Chute channel initiation is observed to be predominantly associated with two of these widening mechanisms; i) an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension, and ii) bank erosion forced by spontaneous mid-channel bar development. The work extends previous empirical analyses, which highlighted the role of bend extension (elongation) in driving chute initiation, with the observation that the frequency of chute initiation increases once bend extension rates and/or widening ratios exceed a reach-scale threshold. A temporal pattern of increased chute initiation frequency on the Ok Tedi, in response to channel steepening and mid-channel bar development following the addition of mine tailings, mirrors the inter- and intra-reach spatial patterns of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power and sediment load are associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy, reminiscent of slope–ratio thresholds in river avulsion. Furthermore, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation may also be rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy (and sediment load), predominantly affecting sites of rapid channel elongation, and thereby mediating the river response.

Spatial and temporal dynamics of sediment accumulation and exchange along Strickland River floodplains (Papua New Guinea) over decadal‐to‐centennial timescales

We investigated the processes of sediment exchange between the Strickland River and its lowland floodplain, documenting (1) the rates, textures, and distributions of sediment accumulation, (2) temporal variations in these rates, (3) the remobilization of sediment by erosive processes, and (4) whether net storage is significant over century timescales. We used 210Pb geochronology of floodplain cores from 11 transects to measure deposition rates over the past ∼65 years, finding a decline from 5.5 cm/a (0–10 m distance from the channel) to 0.9 cm/a (∼400 m) to ∼0.1 cm/a (>1 km). Rates are elevated along curved sections of channel and are stable over time, and episodic accumulation is prevalent and may be correlated to floods. Integrated temporally and spatially, the average rate is 1.6 cm/a. Integrating along both sides of ∼318 river km we studied for the lowland Strickland, we calculate 12–19 Mt of annual sediment accumulation (∼0.05 Mt/km or 0.07% of the annual load per km of river length), representing 17–27% of the total annual sediment flux. We used georeferenced Landsat images (1972–2001) to quantify channel migration and the resulting return of sediment to the channel. Mean lateral migration was 5.1 ± 0.8 m/a. Given the floodplain width of ∼10 km, this implies a waiting time of ∼1 ka between floodplain formation and subsequent reentrainment of the bank as the channel migrates laterally. This raises the possibility that the net return of material from the floodplain due to channel migration could balance the overbank deposition we observed. The exchange flux between cut bank erosion and point bar deposition is 20–40 Mt, highlighting the significance of sediment recycling (∼50% of the total load). Such sediment trapping and recycling affects the transport, storage, and evolution of biogeochemically reactive particles, the evolution of the floodplain, and the morphodynamics of basin infilling.

Net local removal of floodplain sediment by river meander migration

Erosion from the banks of meandering rivers causes a local influx of sediment to the river channel. Most of the eroded volume is usually replaced in nearby point bars. However, the typical geometry of river bends precludes the local replacement of all eroded material because (i) point bars tend to be built to a lower elevation than cutbanks and (ii) point bars tend to be shorter than the eroding portion of cutbanks because of channel curvature. In a floodplain that is in equilibrium (i.e., neither increasing nor decreasing in volume), sediment eroded from cut banks must be replaced elsewhere on the floodplain. The local imbalance caused by differences in bank height should be balanced primarily by overbank deposition, while the local imbalance caused by curvature should be balanced primarily by deposition in abandoned stream courses or oxbow lakes. Estimates of these local imbalances based on remotely sensed measurements of bank geometry and channel migration have been made on four systems in the United States: a 91-km reach of the Pearl River in Louisiana and Mississippi, a 62-km reach of the Bogue Chitto River in Louisiana, a 35-km reach of the Neuse River in North Carolina, and a 2.7-km reach of the Vermillion River in Minnesota. For these systems, the total local imbalance, integrated over many bends, ranged from 0.36 to 2.27 m 3/yr/m of valley length (0.15 to 1.32 m 3/yr/m of channel length), or 7 to 45% of gross cutbank erosion, with a typical value of about 17%. When compared with gauged suspended sediment data, the measurements provide estimates of the relative importance of floodplains for storing material transported by a river system. The data suggest that even if the studied systems are near long-term mass balance equilibrium (as opposed to undergoing net deposition or erosion), almost all of the sand and in some cases much of the silt and clay in transit through these systems is likely to have spent some time stored in an upstream floodplain.

The influence of riparian vegetation on stream width, eastern Pennsylvania, USA

We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active floodplains on the insides of bends. Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E , explain our observations. α represents the influence of grassy vegetation on rates of active floodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/ α. In forested reaches, channels are wide with banks that are difficult to erode. Dense tree roots create a low value of E , and the channel migrates slowly. E/ α is high, however, because α is very low: shade from trees inhibits the growth of grass on active floodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/ α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank floodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation-mediated rates of deposition on active flood-plains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes, rather than focusing on these processes in isolation from one another.

Observed and modeled directional change in riparian forest composition at a cutbank edge

Lateral migrations of river meanders create transient, spatially transgressive edges where the advancing cutbank edge encroaches upon interior floodplain forest communities. This spatial movement of edge toward the forest interior should initiate directional changes in species composition within a forest plot as it is affected by a changing microclimate and hydrological regime. We found that cutbank edge and forest interior sites in an Iowa floodplain contained markedly different plant assemblages. Species commonly associated with later stages of succession dominated interior sites while cutbank edge sites favored secondary, successional species. Assuming that the cutbank edge sites once contained vegetation similar to that surveyed in the floodplain interior, the observed changes in community structure accompanying channel migration are suggestive of retrograde succession, or retrogression. To link cutbank erosional processes with retrogressional processes, we modified a computer simulation model already in use for floodplain environments. We incorporated the changing edge effects and compared model projections with the data collected from the field sites using detrended correspondence analysis. Without changes, the simulation projected a site compositionally similar to the sampled interior forest. When the changes were initiated, the simulated site progressively took on compositional characteristics similar to the riparian edge sites. Because we included only those forcing functions that would be initiated by cutbank erosion, the model supports the hypothesis that the spatially progressive edge effect results in a directional change in forest community composition analogous to retrogression. Our results demonstrate an interesting linkage between successional and fluvial-geomorphic processes and indicate that site dynamics may be controlled differently in landscapes where sites are progressively created and destroyed than where recurrent disturbances affect the same site.

Data Recovery at Justiceburg Reservior (Lake Alan Henry), Garza and Kent Counties, Texas: Phase III, Season 1

Phase III data recovery investigations at one historic and three prehistoric sites, augmented by additional survey and off-site geological investigations, were conducted at Lake Alan Henry (formerly Justiceburg Reservoir) on the Double Mountain Fork of the Brazos River in Garza and Kent counties, Texas, during the fall and winter of 1990-1991. Descriptive data from this first of three field seasons of data recovery are presented here in anticipation of a final synthetic report that will relate all the investigations to a series of research design hypotheses. Analysis of surface distributions at 41KT42, a late nineteenth- to early twentieth-century ranch line camp known locally as the Ed Scott Cabin, groups the artifacts into material and functional categories that demonstrate primary and secondary refuse accumulations adjacent to the cabin. The distribution of primary refuse shows kitchen artifacts are spatially distinct from other types that include architectural, firearm, ranching, clothing, and personal items. The primary discard pattern is obfuscated by redistribution attributed to secondary refuse discard. The uppermost of two superimposed hearths at 41GR484, the Grape Creek Bench Site, is radiocarbon dated to 260 B.P. and yielded charcoal identified as Carya sp. Excavations were halt~d at this Late Prehistoric II site because of extremely low artifact recovery. Geoarcheological investigations demonstrate that most of the site has been destroyed by cut bank erosion associated with the meanderings of Grape Creek. At 41KT33, the Late Prehistoric I South Sage Creek Site, stone-lined hearth features dated to 1005 B.P. are surrounded by clusters of artifacts. Petrographic analysis of brownware ceramics suggests affiliations with the local Palo Duro Complex and nonlocally with the Pecos River valley. The Gobbler Creek Bridge Site, 4IGR383, spans the late Archaic and Late Prehistoric I periods. Intact· and dispersed stone-lined hearths radiocarbon dated to 1865-1215 B.P. are surrounded by artifact clusters. Both of the sites appear to have been multifunctional campsites, and repeated occupations are likely. Dense concentrations of fire-cracked rocks at these sites may represent secondary refuse disposal. Lithic analysis indicates extensive use of nonlocal materials derived from Cretaceous formations that occur in the Callahan Divide and Edwards Plateau to the south. Freshwater mussels are the only faunal remains recovered from either of these sites. The meat is presumed to have been consumed as food, while the shells sometimes were made into ornaments. An additional 440 acres of land were surveyed for cultural resources; 360 of these acres were subsequently acquired by the City of Lubbock for use as wildlife mitigation lands. Nine prehistoric archeological sites and one isolated find were documented and evaluated for their eligibility for listing on the National Register of Historic Places. Five of these sites are considered to be ineligible for listing on the National Register, and five are potentially eligible for listing. Avoidance is recommended at these five sites, with testing recommended to determine their National Register eligibility if avoidance is not possible. Geological investigations provide information about depositional environments in and near the archeological sites. Two soils buried in the Double Mountain Fork floodplain are radiocarbon dated to 8~00 B.P. and 1700-1300 B.P.; fluvial sediments extend as much as 15 m below these soils in portions of the floodplain. An older terrace previously was identified 18 m above the modem river channel. Current work obtained a terminal date of 8690 B.P. from a buried soil that caps this older terrace. Another radiocarbon date of 3320 B.P. was obtained from fill in a gully that is eroded into the older terrace. These dates bracket an erosional episode when the older terrace was heavily dissected. The erosion coincides with the Altithermal, a period believed to have been characterized by a hot and dry climatic regime.

Streambank erosion along two rivers in Iowa

Rates and processes of bank erosion were evaluated for reaches of two alluvial, equiwidth to wide bend, point bar streams in Iowa: East Nishnabotna River and Des Moines River. The evaluation was made using historical records (airphotos, maps, and streamflow records), field measurements, and soil analyses. The erosion rates were correlated with channel characteristics such as width, depth, curvature, arc angle of channel centerline, channel slope, friction factor, and degree of vegetation on the banks. The correlation was studied using a meander flow model developed by the author. The meander flow model simulated the velocity distribution in the bends and the location of the erosion occurrences. Estimates were made of total sediment influx to the rivers from cutbank erosion.

Channel Migration and Incision on the Beatton River

New field data relating to bend migration rates with bend radius of curvature to channel width ratios (rmw) for the Beatton River confirm the form of the relationship the writers described in 1975, although there is more variance than our initial results suggested. Recent data from forest successions on the floodplains of a variety of migrating rivers show that channel migration is a discontinuous process within any single bend. This means that predictions of migration rates for individual bends, based on short‐term measurements using erosion pins or aerial photographs, are highly suspect. These variations in migration rate are likely a response to fluctuating channel widths over time because of a short‐term imbalance between the rate of cutbank erosion and the rate of point‐bar sediment accumulation. Finally the examination of new evidence for channel incision on the Beatton River shows that the river is downcutting at a rate of about 2 mm/yr, not at 10 mm/yr as reported earlier.