Geomorphic Channel Research Articles

Channel-reach morphometric analysis on a headwater stream in a low-mountainous region: a case study from Mecsek Hills

Small catchments in mountainous regions affect downstream rivers as a primary source of sediment supply and also generate flash swasfloods, especially during extreme events. These floods have significantly shaped the catchments of small streams in the Mecsek Hills and some rural areas over the past two decades. However, there has been no previous study examining the hydromorphology of headwater catchments in low mountain environments in Hungary. The present study was meant to investigate the fundamental hydrogeomorphological properties of a first-order catchment. A customary and detailed GIS survey of 50-metre sections was aimed at deciphering flash flood vulnerability and geomorphic interrelations within a micro watershed. We found moderate susceptibility to flash floods compared to the whole Mecsek Hills. Stable large woody debris jams were identified during the field survey as major geomorphic channel features functioning as natural barriers which drive channel evolution and reduce flood hazards.

Dynamics of streamflow permanence in a headwater network: Insights from catchment-scale model simulations

The hillslope and channel dynamics that govern streamflow permanence in headwater systems have important implications for ecosystem functioning and downstream water quality. Recent advancements in process-based, semi-distributed hydrologic models that build upon empirical studies of streamflow permanence in well-monitored headwater catchments show promise for characterizing the dynamics of streamflow permanence in headwater systems. However, few process-based models consider the continuum of hillslope-stream network connectivity as a control on streamflow permanence in headwater systems. The objective of this study was to expand a process-based, catchment-scale hydrologic model to better understand the spatiotemporal dynamics of headwater streamflow permanence and to identify controls of streamflow expansion and contraction in a headwater network. Further, we aimed to develop an approach that enhanced the fidelity of model simulations, yet required little additional data, with the intent that the model might be later transferred to catchments with limited long-term and spatially explicit measurements. This approach facilitated network-scale estimates of the controls of streamflow expansion and contraction, albeit with higher degrees of uncertainty in individual reaches due to data constraints. Our model simulated that streamflow permanence was highly dynamic in first-order reaches with steep slopes and variable contributing areas. The simulated stream network length ranged from nearly 98±2% of the geomorphic channel extent during wet periods to nearly 50±10% during dry periods. The model identified a discharge threshold of approximately 1 mm d−1, above which the rate of streamflow expansion decreases by nearly an order of magnitude, indicating a lack of sensitivity of streamflow expansion to hydrologic forcing during high-flow periods. Overall, we demonstrate that process-based, catchment-scale models offer important insights on the controls of streamflow permanence, despite uncertainties and limitations of the model. We encourage researchers to increase data collection efforts and develop benchmarks to better evaluate such models.

Controls on Stream Water Age in a Saturation Overland Flow‐Dominated Catchment

AbstractWater age and flow pathways should be related; however, it is still generally unclear how integrated catchment runoff generation mechanisms result in streamflow age distributions at the outlet. Here, we combine field observations of runoff generation at the Dry Creek catchment with StorAge Selection (SAS) age models to explore the relationship between stream water age and runoff pathways. Dry Creek is a 3.5 km2catchment in the Northern California Coast Ranges with a Mediterranean climate, and, despite an average rainfall of ≈1,800 mm/yr, is an oak savannah due to the limited hillslope water storage capacity. Runoff lag to peak—after initial seasonal wet‐up—is rapid (∼1–2 hr), and total annual streamflow consists predominantly of saturation overland flow, based on field mapping of saturated extents and an inferred runoff threshold for the expansion of saturation extent beyond the geomorphic channel. SAS modeling based on daily isotope sampling reveals that streamflow is typically older than 1 day. Since streamflow primarily consists of overland flow, a significant portion of overland flow must not be event‐rain but instead derive from older, nonevent groundwater returning to the surface, consistent with field observations of exfiltrating head gradients, return flow through macropores, and extensive saturation days after storm events. We conclude that even in a watershed fed primarily by overland flow, runoff is primarily not composed of event water. Our findings have implications for the interpretation of stream chemistry and the assumptions built into widely used hydrograph separation inferences, namely, the assumption that overland flow consists of new (event) water.

Landscape geomorphology and local‐riverine features influence Broad Whitefish ( Coregonus nasus ) spawning habitat suitability in Arctic Alaska

Landscape-level geomorphic processes influence the spatial and temporal arrangement of fish habitats in freshwater ecosystems and fishes move across riverscapes, selecting a suite of habitats to maximise fitness. Here, we explore the influence of geomorphology on stream channel attributes and assess Broad Whitefish (Coregonus nasus) spawning habitat potential in the Colville River in Arctic Alaska. Using high-resolution digital surface models (5 m2), we quantified the stream network extent and summarised channel habitat attributes continuously across the drainage network. Next, we developed an intrinsic potential (IP) model for Broad Whitefish by using geomorphic channel parameters previously understood to be associated with spawning habitats (channel width, median substrate size and channel braiding) to estimate the potential of streams across the Colville River watershed to provide spawning habitat. Our model results show the majority of habitat with high IP (≥0.6) was located within the braided sections of the main channel, which encompass >1548 km, but only 2% of the total channel network. The IP model was tested by tracking radio-tagged Broad Whitefish using aerial surveys. Prespawn fish moved into the watershed starting mid-July and mostly used habitat with moderate to very high IP in the middle and lower watershed. Several individuals were relocated in smaller multichannels with vegetated bars that contained very low IP (≤0.2), suggesting that other factors, such as hyporheic flow, may also influence spawning habitat selection. Our study demonstrates that IP modelling offers a useful method to quantify spawning habitat potential in data-poor riverscapes, providing useful information for managers to assess potential anthropogenic impacts and develop conservation plans to protect essential Broad Whitefish habitat.

High-resolution remote sensing and multistate occupancy estimation identify drivers of spawning site selection in fall chum salmon (Oncorhynchus keta) across a sub-Arctic riverscape

Groundwater upwellings provide warmer, stable overwinter temperatures for developing salmon embryos, which may be particularly important in cold, braided, gravel-bed sub-Arctic rivers. We used a three-year time series of aerial counts and remote sensing to estimate the distribution of low and high aggregations of spawning fall chum salmon (Oncorhynchus keta), classify approximately 0.5 km long river segments by geomorphic channel type, and map thermal variability along a 25.4 km stretch of the Teedriinjik River, Alaska. We used a dynamic multistate occupancy model to estimate detectability, occupancy, and the dynamics of spawning aggregations among river segments. Detectability was higher for large (>150) relative to smaller aggregations. Unoccupied segments were likely to remain so from year to year; low abundance spawning segments were dynamic and rarely remained in that state for multiple years, while ∼20%–35% of high abundance segments remained stable, indicating the presence of high-quality spawning habitat. Spawning habitat use was associated with warmer water temperatures likely caused by groundwater upwellings. We identified spawning habitat characteristics and trends in usage by fall chum salmon, which will inform land management decisions and assist in evaluating impacts of shifting climate conditions and resource management on Arctic salmon populations.

Evaluating the geomorphic channel response to beaver dam analog installation using unoccupied aerial vehicles

AbstractBeaver dam analogs (BDAs) are a stream restoration technique that is rapidly gaining popularity in the western United States. These low‐cost, stream‐spanning structures, designed after natural beaver dams, are being installed to confer the ecologic, hydrologic, and geomorphic benefits of beaver dams in streams that are often too degraded to provide suitable beaver habitat. BDAs are intended to slow streamflow, reduce the erosive power of the stream, and promote aggradation, making them attractive restoration tools in incised channels. Despite increasing adoption of BDAs, few studies to date have monitored the impacts of BDAs on channel form. Here, we examine the geomorphic changes that occurred within the first year of restoration efforts in Wyoming using high‐resolution visible light orthomosaics and elevation data collected with unoccupied aerial vehicles (UAVs). By leveraging the advantages of rapidly acquired images from UAV surveys with recent advancements in structure‐from‐motion photogrammetry, we constructed centimeter‐scale digital elevation models (DEMs) of the restoration reach and an upstream control reach. Through DEM differencing, we identified areas of enhanced erosion and deposition near the BDAs, suggesting BDA installation initiated a unique geomorphic response in the channel. Both reaches were characterized by net erosion during the first year of restoration efforts. While erosion around the BDAs may seem counter to the long‐term goal of BDA‐induced aggradation, short‐term net erosion is consistent with high precipitation during the study and with theoretical channel evolution models of beaver‐related stream restoration that predict initial channel widening and erosion before net deposition. To better understand the impacts of BDAs on channel morphology and restoration efforts in the western United States, it is imperative that we consistently assess the effects of beaver‐inspired restoration projects across a range of hydrologic and geomorphic settings and that we continue this monitoring in the future.

Machine Learning Predicts Reach‐Scale Channel Types From Coarse‐Scale Geospatial Data in a Large River Basin

AbstractHydrologic and geomorphic classifications have gained traction in response to the increasing need for basin‐wide water resources management. Regardless of the selected classification scheme, an open scientific challenge is how to extend information from limited field sites to classify tens of thousands to millions of channel reaches across a basin. To address this spatial scaling challenge, this study leverages machine learning to predict reach‐scale geomorphic channel types using publicly available geospatial data. A bottom‐up machine learning approach selects the most accurate and stable model among 20,000 combinations of 287 coarse geospatial predictors, preprocessing methods, and algorithms in a three‐tiered framework to (i) define a tractable problem and reduce predictor noise, (ii) assess model performance in statistical learning, and (iii) assess model performance in prediction. This study also addresses key issues related to the design, interpretation, and diagnosis of machine learning models in hydrologic sciences. In an application to the Sacramento River basin (California, USA), the developed framework selects a Random Forest model to predict 10 channel types previously determined from 290 field surveys over 108,943 two hundred‐meter reaches. Performance in statistical learning is reasonable with a 61% median cross‐validation accuracy, a sixfold increase over the 10% accuracy of the baseline random model, and the predictions coherently capture the large‐scale geomorphic organization of the landscape. Interestingly, in the study area, the persistent roughness of the topography partially controls channel types and the variation in the entropy‐based predictive performance is explained by imperfect training information and scale mismatch between labels and predictors.

Evaluation of the removal of impassable barriers on anadromous salmon and steelhead in the Columbia River Basin

AbstractDespite the popularity of barrier removal as a habitat restoration technique, there are few studies that evaluate the biological effects of restored stream crossings. An extensive post‐treatment study design was used to quantify fish populations (e.g. species, life stage, abundance) and habitat attributes (e.g. gradient, geomorphic channel units) at 32 culvert removal or replacement projects to determine their effectiveness in restoring habitat access for juvenile salmon, Oncorhynchus spp., and steelhead, O. mykiss (Walbaum), in the Columbia River Basin, USA. Anadromous fish (steelhead, Chinook salmon O. tshawytscha [Walbaum]) abundance, juvenile steelhead abundance and habitat conditions were not significantly different between paired reaches (i.e. upstream and downstream of former barrier sites), suggesting these sites are no longer full barriers to movement. This suggests that barrier removal projects on small Columbia Basin streams provide adequate fish passage, increased habitat availability and increased juvenile anadromous fish abundance immediately upstream of former barriers.

River sensitivity and sediment connectivity as tools for assessing future geomorphic channel behavior

ABSTRACT The science of geomorphology is increasingly used to inform river management efforts; however, the complexity of fluvial systems makes predictions of future channel adjustment difficult at best. The geomorphic concepts of landform sensitivity and sediment connectivity are well suited to aid river managers in assessing the probability and variability of river channel responses. This can be especially helpful in planning for impacts of future climate change or changes in management activity. River sensitivity and sediment connectivity datasets provide a necessary reach-in-catchment perspective to inform geomorphic interpretations of river behaviour. Such interpretations constitute a first step towards providing the contextual geomorphic understanding necessary to establish expectations of future channel behaviour. In this paper, we use geomorphic interpretations based on river sensitivity and sediment connectivity datasets to describe the historical trajectory and future adjustment possibilities for four channel reaches in the Lockyer Valley, southeast Queensland (SEQ). We apply our interpretations to three different scenarios of future climate and river management conditions to constrain what forms of geomorphic adjustment are possible for resilient-disconnected, resilient-connected, sensitive-connected, and sensitive-disconnected channels. Using scenario-building exercises to forecast possibilities of river adjustment can aid river managers by establishing expectations of future channel behaviour. This information can then be fed into the decision-making process regarding where to prioritize management actions as part of catchment action planning that works with, not against, the natural behaviour of riverine environments.

An automatic and objective approach to hydro-flatten high resolution topographic data

Hydro-flattening is an operation required to generate deliverable terrain products after lidar survey collections that usually entails extensive manual intervention. In this paper, we develop new modules of GeoNet, a computational tool for the automatic extraction of geomorphic channel features from high resolution topography, for detecting channel banks and produce a hydro-flattened Digital Terrain Model (DTM). We first review the original GeoNet workflow and then describe how it has been modified to extract channels as the least-cost-paths from given channel heads to outlets. A new code component enables hydro-flattening on a given reference network based on curvature and connectivity. A raster-based routine for extracting the geomorphic channel zone based on a statistical analysis of slope has also been added. We test our new components using three different test cases. Compared to manually delineated hydro-flattened zones and satellite imagery, our results show high consistency and the capability of our method to automatically hydro-flatten high resolution topographic data.

Morphology and channel characteristics of an equatorial tropical river in Malaysian Borneo: a detailed evaluation through spatially explicit geomorphometric modelling

Geomorphic and morphodynamic characteristics of Akah River in Sarawak, Malaysia in north-western Borneo were modelled using geographical information system assisted techniques and geomorphometric indices. Stream properties such as longitudinal profile, stream length gradient index (SL), channel slope, hypsometric curve, hypsometric integral, sinuosity index and channel cross profiles were analyzed. Morphological features such as meander and channel bar characteristics were evaluated and compared to classify the river and to identify the processes responsible for its present nature. The longitudinal profile, SL index and channel slope indicate the presence of multiple knick points in the channel, which may be indicative of changes in the base level characteristics of the river or lithological differences. The characteristic convex shape of hypsometric toe indicates late stage of deposition due to diffusive processes and the hypsometric integral identifies the geomorphic evolution as in the advanced stage i.e. erosion is significant. The sinuosity index classifies the entire river as meandering and some segments as tortuously meandering with varied stream valleys. Numerous channel bars of various dimensions are found in different reaches of the stream indicating varying channel bottom characteristics and flow mechanism due to the influence of geological process. The information generated from spatial modelling of different geomorphic indices (different terrain variables) and channel characteristics classifies the Akah subwatershed with slope (diffusive) and fluvial dominated geomorphic process having highly meandering channel and showing anomalous characteristics in some segments. These findings will serve as baseline information, while considering the region for hydrological development activities which include check dams, mini hydroelectric projects.

Headwater stream length dynamics across four physiographic provinces of theAppalachianHighlands

AbstractUnderstanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in 3 forested catchments (<75 ha) in each of 4 physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length 7 times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.

Channel Geomorphic Evolution after Dam Removal: Is Scale Important?

Post–dam removal geomorphologic adjustment of a stream channel has been documented in the scientific literature at watershed, hillslope, and laboratory scales. Hillslope-scale studies in channel cross sections are most common and add significant value in the dam-removal literature. This study examines geomorphic stream channel adjustment following dam removal at the hillslope scale under natural climatic conditions. A sediment-filled silt fence dam (1 m tall, 12.65 m wide) was removed in three stages, and the width and depth of the upstream developing channel was monitored at six transects for 15 months. Headcut retreat and changes in channel sinuosity were also recorded. After the silt fence dam was removed, channel development was initiated by headcut formation, which migrated upstream at a rate of 4 cm/d for about 10 months and then gradually reached attenuation. The channel progressed through four distinct stages: Stage 1 (Initial conditions); Stage 2 (Downcutting)—wide, shallow, meandering channel incised to a maximum depth of 0.52 m, and sinuosity decreased; Stage 3 (Floodplain development)—upon reaching base level, surface runoff began to meander within the channel, widening it through bank slumps and erosion; and Stage 4 (Quasi-equilibrium)—channel development reached dynamic (quasi-) equilibrium with only minor widening at downstream transects (maximum width of the incised channel reached 0.46 m), accompanied by sediment aggradation. The stages of upstream channel development and headcut retreat pattern in this study are consistent with the findings of other studies at the laboratory and watershed scales, indicating that channel development after dam removal is scale independent.

Comparing submarine and fluvial channel kinematics: Implications for stratigraphic architecture

Abstract Submarine and fluvial channels exhibit qualitatively similar geomorphic patterns, yet produce very different stratigraphic records. We reconcile these seemingly contradictory observations by focusing on the channel belt scale and quantifying the time-integrated stratigraphic record of the belt as a function of the scale and trajectory of the geomorphic channel, applying the concept of stratigraphic mobility. By comparing 297 submarine and fluvial channel belts from a range of tectonic settings and time intervals, we identify channel kinematics (trajectory) rather than channel morphology (scale) as the primary control on stratigraphic architecture and show that seemingly similar channel forms (in terms of scaling) have the potential to produce markedly different stratigraphy. Submarine channel belt architecture is dominated by vertical accretion (aggradational channel fill deposits), in contrast to fluvial systems that are dominated by lateral accretion (point bar deposits). This difference is best described with the channel belt aspect ratio, which is 9 for submarine systems and 72 for fluvial systems. Differences in channel kinematics and thus stratigraphic architecture between the two environments appear to result from markedly different coupling between channel aggradation and overbank deposition. The methodology and results presented here are also applicable to interpreting channelized stratigraphy on other planets and moons.

The classical turbidite outcrop at San Clemente, California revisited: An example of sandy submarine channels with asymmetric facies architecture

A 1.1–1.2km long, 3–15m thick exposure of the late Miocene to Pliocene Capistrano Formation crops out at San Clemente, California, providing a superb example of submarine channel elements with an asymmetric cross-sectional facies distribution. Coarser-grained, thicker bedded and more amalgamated channel axial deposits are partitioned towards one side of channel elements (200–400m wide), whilst finer-grained and thinner bedded channel margin deposits are partitioned towards the other side. Two end-member types of silty channel-base and intra-channel drapes are recognized, namely, bypass drapes and deposition drapes. There are both draping silty turbidites that show either strong (bypass drapes) or insignificant (deposition drapes) evidence of erosion and/or sediment bypass during deposition. Bypass drapes and deposition drapes are interpreted to result from flow bypass and flow stratification, respectively, and have significantly different implications for reservoir connectivity and down-dip sediment transport. Channel elements are nested to form two channel complexes. Channel complex 1 comprises four channel elements and shows a vertical aggradation dominated stacking pattern, whilst channel complex 2 comprises five channel elements and shows a mixed lateral migration/vertical aggradation stacking pattern. This study also suggests that these exposures represent only a fragment of a larger channel complex set that might bear varying degrees of resemblance to its formative geomorphic channel(s) on the paleo-seafloor. The reinterpretation of this classic outcrop provides valuable insight into other turbidite channel systems at outcrop and in the subsurface, both in a sedimentological and applied context.

Stream network geomorphology mediates predicted vulnerability of anadromous fish habitat to hydrologic change in southeast Alaska.

In rivers supporting Pacific salmon in southeast Alaska, USA, regional trends toward a warmer, wetter climate are predicted to increase mid- and late-21st-century mean annual flood size by 17% and 28%, respectively. Increased flood size could alter stream habitats used by Pacific salmon for reproduction, with negative consequences for the substantial economic, cultural, and ecosystem services these fish provide. We combined field measurements and model simulations to estimate the potential influence of future flood disturbance on geomorphic processes controlling the quality and extent of coho, chum, and pink salmon spawning habitat in over 800 southeast Alaska watersheds. Spawning habitat responses varied widely across watersheds and among salmon species. Little variation among watersheds in potential spawning habitat change was explained by predicted increases in mean annual flood size. Watershed response diversity was mediated primarily by topographic controls on stream channel confinement, reach-scale geomorphic associations with spawning habitat preferences, and complexity in the pace and mode of geomorphic channel responses to altered flood size. Potential spawning habitat loss was highest for coho salmon, which spawn over a wide range of geomorphic settings, including steeper, confined stream reaches that are more susceptible to streambed scour during high flows. We estimated that 9-10% and 13-16% of the spawning habitat for coho salmon could be lost by the 2040s and 2080s, respectively, with losses occurring primarily in confined, higher-gradient streams that provide only moderate-quality habitat. Estimated effects were lower for pink and chum salmon, which primarily spawn in unconfined floodplain streams. Our results illustrate the importance of accounting for valley and reach-scale geomorphic features in watershed assessments of climate vulnerability, especially in topographically complex regions. Failure to consider the geomorphic context of stream networks will hamper efforts to understand and mitigate the vulnerability of anadromous fish habitat to climate-induced hydrologic change.

The long-term impact of channel stabilization using gabion structures on Zealand River, New Hampshire

Zealand River, NH contains the second oldest major stream-stabilization project in the U.S. that extensively used gabions, which are stone-filled, wire-mesh baskets used to construct revetment walls, grade-control sills, or groin deflectors. In 2014, a study was conducted on a 4.5-km stretch of river to determine the status of gabion structures installed from 1960 to 1963, and the impact of those gabions on geomorphic channel stability. Longitudinal profiles, cross-sectional surveys and field observations provide evidence of channel incision, narrowing and avulsions at collapsed walls. Gabion sills failed first, which allowed 1–2m of localized incision that undercut gabion walls, which then toppled into the eroded channel. Corrosion and abrasion by bedload movement, floating large wood and winter ice enhanced failure of gabion structures by breaking wire at the base of walls. Gabions with broken wires often spilled their rock fill and lost their structural integrity. Although gabions were intended to stabilize the river, they enhanced vertical channel incision, failed to prevent bank instability, and created localized channel widening and avulsions associated with depositional reaches.

Catchment‐ and reach‐scale controls on the distribution and expectation of geomorphic channel adjustment

AbstractVariability in channel function (behavior) can be assessed by characterizing different forms of adjustment over time. Here, historical channel adjustments in three tributary systems of the Lockyer Valley, Southeast Queensland (SEQ) are analyzed in order to evaluate the range of catchment‐ and reach‐scale controls on channel behavior. Over 300 individual adjustments and 13 forms of adjustment were identified over a ∼130 year time span. We measured the width‐to‐depth ratio (W:D), mean stream power (ω), and basin area (A) at the location of all observed adjustments. The most common forms of adjustment were avulsions, lateral expansion of the channel, and bend adjustments. The tributary systems behave distinctly different from one another according to statistical comparisons between the W:D, ω, and A data for these forms of adjustment. We find that it is possible to develop process domains or typologies for forms of geomorphic adjustment found in the Lockyer Valley. These domains or typologies provide the foundations for synoptic comparisons between catchments and assessing the expectation of channel adjustment (forecasting), which should be included in process‐based river management practice.

The geomorphic legacy of small dams—An Austrian study

Dams represent one of the most dominant forms of human impact upon fluvial systems during the Anthropocene, as they disrupt the downstream transfer of water and sediments. Removing dams restores river continuity and channel morphology. Both dam construction and dam removal induce geomorphic channel responses that often require the installation of channel protection structures. Although such measures are well-established in river engineering, little is known about their interactions or legacy effects on river sediment dynamics, channel morphology and riverine habitats. This study investigated the legacy effects of small dams and their removal on bed sediment and channel morphology in two small mixed-load streams in Austria using field mapping and DEM-based geomorphometric channel analyses. At active dams, results showed increases in channel slope (1.76–13.88%) and depth (0.1–2 m) as well as in dominant bed sediment grain size. At some dam removal sites without channel protection structures, we observed balanced channel slope conditions and decreases in channel depth (0.5–1.9 m) as well as homogeneous bed sediment textures. At sites exhibiting channel engineering, bed and bank protection structures inhibited geomorphic response to dam removal, thereby preserving dam-induced channel conditions. However, at numerous locations geomorphic responses to dams and their removal were observed to be more complex as they are governed by dam interactions and feedback processes that are further influenced by the proximity of other dams and related hydro-geomorphic conditions.

The influence of spatially variable stream hydraulics on reach scale transient storage modeling

AbstractWithin the context of reach scale transient storage modeling, there is limited understanding of how best to establish reach segment lengths that represent the effects of spatially variable hydraulic and geomorphic channel properties. In this paper, we progress this understanding through the use of channel property distributions derived from high‐resolution imagery that are fundamental for hydraulic routing. We vary the resolution of reach segments used in the model representation and investigate the minimum number necessary to capture spatially variable influences on downstream predictions of solute residence time probability density functions while sufficiently representing the observed channel property distributions. We also test if the corresponding statistical moments of the predictions provide comparable results and, therefore, a method for establishing appropriate reach segment lengths. We find that the predictions and the moment estimates begin to represent the majority of the variability at reach segment lengths coinciding with distances where observed channel properties are spatially correlated. With this approach, reach scales where the channel properties no longer significantly change predictions can be established, which provides a foundation for more focused transient storage modeling efforts.