Assessment of spatial variation of cesium-137 in small catchments

Surface contamination by bomb-derived and Chernobyl-derived 137Cs has been subject to changes due to physical decay and lateral transport of contaminated soil particles, which have resulted in an on-going transfer of radionuclides from terrestrial ecosystems to surface water, river bed sediments, and flood plains. Knowledge of the different sources of spatial variation of 137Cs is particularly essential for estimating 137Cs transfer to fluvial systems and for successfully applying 137Cs as an environmental tracer in soil erosion studies. This study combined a straightforward sediment redistribution model and geostatistical interpolation of point samples of 137Cs activities in soil to distinguish the effects of sediment erosion and deposition from other sources of variation in 137Cs in the small Mochovce catchment in Slovakia. These other sources of variation could then be interpreted. Besides erosion and deposition processes, the initial pattern of 137Cs deposition, floodplain sedimentation, and short-range spatial variation were identified as the major sources of spatial variation of the 137Cs inventory.     

Management of Sedimentation in Tropical Watersheds

/ The sedimentation of reservoirs is a serious problem throughout the tropics, yet most attempts to control sedimentation in large river basins have not been very successful. Reliable information on erosion rates and sources of sediments has been lacking. In regions where geologically unstable terrain combines with high rainfall, natural erosion rates might be so high that the effects of human activity are limited. Estimates of natural erosion in these situations often have been poor because of the episodic nature of most erosion during large storms and because mass-wasting may supply much of the sediment. The predominance of mass-wasting in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedimentation to "live" rather than "dead" storage within reservoirs. Furthermore, the inappropriate use of the Universal Soil Loss Equation to assess the effectiveness of erosion control measures has led to inaccurate estimates of the sediment reduction benefits that could accrue to watershed treatment efforts. Although reducing erosion from cultivated areas is desirable for other reasons, efforts aimed at reducing reservoir sedimentation by controlling agricultural sources of erosion may have limited benefits if the principal sources are of natural origin or are associated with construction of the dams and reservoirs and with rural roads and trails. Finally, the most appropriate locations for watershed rehabilitation depend on the magnitude of temporary storage of colluvium and alluvium within the river basin: Where storage volume is large and residence time of sediment very long, reducing agricultural erosion may have limited impacts on sedimentation within the expected life of a reservoir. Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resources towards reducing sedimentation. When sedimentation of reservoirs is the key issue, sediment budgets must focus especially on channel transport rates and sediment delivery from hillsides. Sediment budgets are especially critical for tropical areas where project funds and technical help are limited. Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective. KEY WORDS: Tropical watersheds; Sedimentation; Reservoirs; Erosion control     

PROFILE: Hungry Water: Effects of Dams and Gravel Mining on River Channels

/ Rivers transport sediment from eroding uplands to depositional areas near sea level. If the continuity of sediment transport is interrupted by dams or removal of sediment from the channel by gravel mining, the flow may become sediment-starved (hungry water) and prone to erode the channel bed and banks, producing channel incision (downcutting), coarsening of bed material, and loss of spawning gravels for salmon and trout (as smaller gravels are transported without replacement from upstream). Gravel is artificially added to the River Rhine to prevent further incision and to many other rivers in attempts to restore spawning habitat. It is possible to pass incoming sediment through some small reservoirs, thereby maintaining the continuity of sediment transport through the system. Damming and mining have reduced sediment delivery from rivers to many coastal areas, leading to accelerated beach erosion. Sand and gravel are mined for construction aggregate from river channel and floodplains. In-channel mining commonly causes incision, which may propagate up- and downstream of the mine, undermining bridges, inducing channel instability, and lowering alluvial water tables. Floodplain gravel pits have the potential to become wildlife habitat upon reclamation, but may be captured by the active channel and thereby become instream pits. Management of sand and gravel in rivers must be done on a regional basis, restoring the continuity of sediment transport where possible and encouraging alternatives to river-derived aggregate sources.KEY WORDS: Dams; Aquatic habitat; Sediment transport; Erosion; Sedimentation; Gravel mining     

Modeling Pre‐ and Post‐Dam Removal Sediment Dynamics: The Kalamazoo River,Michigan1

Abstract:  The state of Michigan is interested in removing two low‐head dams in an 8.8 km reach of the Kalamazoo River between Plainwell and Otsego, Michigan, while minimizing impacts locally and to downstream reaches. The study was designed to evaluate the erosion, transport, and deposition of sediments over a 37.3‐year period using the channel evolution model CONCEPTS for three simulation scenarios: Dams In (DI), Dams Out (DO), and Design (D). The total mass of sediment emanating from the channel boundary, for the DI case, shows net deposition of 4,100 T/y for the study reach, with net transport (suspended and bed load) of 10,500 T/y passing the downstream boundary. For the DO case, net erosion is 19,200 T/y with net transport of 30,100 T/y (187% increase) passing the downstream boundary. For the D case, net deposition is 2,570 T/y (37% decrease) with transport of 14,200 T/y (35% increase) passing the downstream boundary. The most significant findings were: (1) removal of the low‐head dams will cause significant erosion of sediments stored behind the dams and increased sediment loads passing the downstream boundary and (2) sediment loads for the proposed channel design are similar to existing conditions and offer reduced fine‐sediment loadings.     

Modeling Flow and Sediment Transport in a River System Using an Artificial Neural Network

A river system is a network of intertwining channels and tributaries, where interacting flow and sediment transport processes are complex and floods may frequently occur. In water resources management of a complex system of rivers, it is important that instream discharges and sediments being carried by streamflow are correctly predicted. In this study, a model for predicting flow and sediment transport in a river system is developed by incorporating flow and sediment mass conservation equations into an artificial neural network (ANN), using actual river network to design the ANN architecture, and expanding hydrological applications of the ANN modeling technique to sediment yield predictions. The ANN river system model is applied to modeling daily discharges and annual sediment discharges in the Jingjiang reach of the Yangtze River and Dongting Lake, China. By the comparison of calculated and observed data, it is demonstrated that the ANN technique is a powerful tool for real-time prediction of flow and sediment transport in a complex network of rivers. A significant advantage of applying the ANN technique to model flow and sediment phenomena is the minimum data requirements for topographical and morphometric information without significant loss of model accuracy. The methodology and results presented show that it is possible to integrate fundamental physical principles into a data-driven modeling technique and to use a natural system for ANN construction. This approach may increase model performance and interpretability while at the same time making the model more understandable to the engineering community.     

Temporal and spatial distributions of sediment total organic carbon in an estuary river

Understanding temporal and spatial distributions of naturally occurring total organic carbon (TOC) in sediments is critical because TOC is an important feature of surface water quality. This study investigated temporal and spatial distributions of sediment TOC and its relationships to sediment contaminants in the Cedar and Ortega Rivers, Florida, USA, using three-dimensional kriging analysis and field measurement. Analysis of field data showed that large temporal changes in sediment TOC concentrations occurred in the rivers, which reflected changes in the characteristics and magnitude of inputs into the rivers during approximately the last 100 yr. The average concentration of TOC in sediments from the Cedar and Ortega Rivers was 12.7% with a maximum of 22.6% and a minimum of 2.3%. In general, more TOC accumulated at the upper 1.0 m of the sediment in the southern part of the Ortega River although the TOC sedimentation varied with locations and depths. In contrast, high concentrations of sediment contaminants, that is, total polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), were found in sediments from the Cedar River. There was no correlation between TOC and PAHs or PCBs in these river sediments. This finding is in contradiction to some other studies which reported that the sorption of hydrocarbons is highly related to the organic matter content of sediments. This discrepancy occurred because of the differences in TOC and hydrocarbon source input locations. It was found that more TOC loaded into the southern part of the Ortega River, while almost all of the hydrocarbons entered into the Cedar River. This study suggested that the locations of their input sources as well as the land use patterns should also be considered when relating hydrocarbons to sediment TOC.     

Initial Adjustments Within a New River Channel: Interactions Between Fluvial Processes, Colonizing Vegetation, and Bank Profile Development

A conceptual model of the morphological development of the riparian margins of newly cut river channels is presented, suggesting early feedbacks between vegetation growth and bank form. To test the model, observations of long and cross profiles, bank sediment and seed deposition, and bank vegetation development were collected over the first 2 years of river flows through a reach of the River Cole, West Midlands, UK. The newly created channel had a sinuous planform and varying asymmetric trapezoidal cross section in sympathy with the planform. No imposed bedforms or bank reseeding were included in the design. Over the 2 years, development of bedforms was rapid, with bed sediment sorting and bank profile adjustment occurring more steadily and progressively. Six classes of bank profile were identified by the end of the study period, illustrating close associations with sediment aggradation, vegetation colonization, and growth patterns. Vegetation colonization of the banks was seeded predominantly from local sources during the summer and from hydrochory (transport by the river) during the winter. Colonizing vegetation on the riverbanks appeared to act as a significant propagule source by the second summer and as an increasingly important roughness element, trapping both propagules and sediment, within the second year and providing early feedback into bank evolution. As a result, the time required for riparian margin development in the conceptual model was found to be considerably longer than observed in the study river. In addition, the role of surface wash/bank failure in modifying the bank profile and transporting seeds onto the upper bank face during the first year of bank development was found to be important in initiating rapid bank vegetation colonization and surface stabilization. This set of processes had not been incorporated in the initial conceptual model. In relation to channel restoration, this research illustrates that in small temperate rivers of modest energy the provision of an initial, sinuous corridor is sufficient to induce rapid development of fluvial features and vegetation cover without the need to construct bed forms or to seed the banks.     

Using Sediment Budgets to Investigate the Pathogen Flux Through Catchments

We demonstrate a materials budget approach to identify the main source areas and fluxes of pathogens through a landscape by using the flux of fine sediments as a proxy for pathogens. Sediment budgets were created for three subcatchment tributaries of the Googong Reservoir in southeastern New South Wales, Australia. Major inputs, sources, stores, and transport zones were estimated using sediment sampling, dam trap efficiency measures, and radionuclide tracing. Particle size analyses were used to quantify the fine-sediment component of the total sediment flux, from which the pathogen flux was inferred by considering the differences between the mobility and transportation of fine sediments and pathogens. Gullies were identified as important sources of fine sediment, and therefore of pathogens, with the pathogen risk compounded when cattle shelter in them during wet periods. The results also indicate that the degree of landscape modification influences both sediment and pathogen mobilization. Farm dams, swampy meadows and glades along drainage paths lower the flux of fine sediment, and therefore pathogens, in this landscape during low-flow periods. However, high-rainfall and high-flow events are likely to transport most of the fine sediment, and therefore pathogen, flux from the Googong landscape to the reservoir. Materials budgets are a repeatable and comparatively low-cost method for investigating the pathogen flux through a landscape.     

A geomorphologist's criticism of the engineering approach to channelization of gravel-bed rivers: case study of the Raba River, Polish Carpathians

River engineers use sediment transport formulas to design regulated channels in which the river's ability to transport bedload would remain in equilibrium with the delivery of materials from upstream. In gravel-bed rivers, a number of factors distort the simple relationship between particle size and hydraulic parameters at the threshold of sediment motion, inherent in the formulas. This may lead to significant errors in predicting the bedload transport rates in such streams and hence to instability of their regulated channels. The failure to recognize a nonstationary river regime may also result in unsuccessful channelization. Rapid channel incision has followed channelization of the main rivers of the Polish Carpathians in the 20th century. A case study of the Raba River shows that incision has resulted from the increase in stream power caused by channelization and the simultaneous reduction in sediment supply due to variations in basin management and a change in flood hydrographs. Calculations of bedload transport in the river by the Meyer-Peter and Müller formula are shown to have resulted in unrealistic estimates, perhaps because the different degree of bed armoring in particular cross-sections was neglected. It would have been possible to avoid improper channelization if the decreasing trend in sediment load of the Carpathian rivers had been recognized on the basis of geomorphological and sedimentological studies. Allowing the rivers to increase their sinuosity, wherever possible without an erosional threat to property and infrastructure, and preventing further in-stream gravel mining are postulated in order to arrest channel incision and reestablish the conditions for water and sediment storage on the floodplains.     

Tin mining and sediment supply in peninsular malaysia with special reference to the Kelang River Basin

Summary The paper discusses the tin mining industry in Malaysia and legislations promulgated to prevent environmental degradation due to mining activities. Soil erosion and sediment contribution from mining areas were studied by examining inflow and outflow sediment loads and stormflow sediment transport characteristics at two sediment sampling stations in the Kelang River Basin. It was observed that despite the cessation of mining activities, rivers draining mining areas still carry high sediment loads. It was found that sediment yield increases by three to six times after the river passes through mining land. Low-flow sediment transport is more significant in the mining area compared to non-mining areas. Clockwise hytheretical loops observed at the two sampling stations indicate that sediment sources are located near the river and within the channel system itself.Mr G. Balamurugan was until recently a member of staff at the Institute for Advanced Studies at the University of Malaya. He is now a practising water resources engineer, and may be contacted at HSS Integrated, 20–24, Jalan SS 21/62, Damansara Utama, 47400 Petaling Jaya, Selangor, Malaysia.     

Backwater Confluences of the Ohio River: Organic and Inorganic Fingerprints Explain Sediment Dynamics in Wetlands and Marinas

In the Ohio River (OR), backwater confluence sedimentation dynamics are understudied, however, these river features are expected to be influential on the system’s ecological and economic function when integrated along the river’s length. In the following paper, we test the efficacy of organic and inorganic tracers for sediment fingerprinting in backwater confluences; we use fingerprinting results to evidence sediment dynamics controlling deposition patterns in confluences used for wetland and marina functions; and we quantify the spatial extent of tributary drainages with wetland and marina features in OR confluences. Both organic and inorganic tracers statistically differentiate sediment from stream and river end‐members. Carbon and nitrogen stable isotopes produce greater uncertainty in fingerprinting results than inorganic elemental tracers. Uncertainty analysis of the nonconservative tracer term in the organic matter fingerprinting application estimates an apparent enrichment of the carbon stable isotopes during instream residence, and the nonconservativeness is quantified with a statistical approach unique to the fingerprinting literature. Wetland and marina features in OR confluences impact 42% and 11% of tributary drainage areas, respectively. Sediment dynamics show wetland and marina confluences experience deposition from river backwaters with longitudinally linear and nonlinear patterns, respectively, from sediment sources.     

A Decade of Geomorphic and Hydraulic Response to the La Valle Dam Project,Baraboo River,Wisconsin

We investigate stream response to the La Valle Dam removal and channel reconstruction by estimating channel hydraulic parameter values and changes in sedimentation within the reservoir. The designed channel reconstruction after the dam removal included placement of a riffle structure at the former dam site. Stream surveys undertaken in 1984 by Federal Emergency Management Agency and in 2001 by Doyle et al. were supplemented with surveys in 2009 and 2011 to study the effects of the instream structure. We created a model in HEC‐RAS IV and surface maps in Surfer© using the 1984, 2009, and 2011 surveys. The HEC‐RAS IV model for 2009 channel conditions indicates that the riffle structure decreases upstream channel shear stress and velocity, causing renewed deposition of sediment within the former reservoir. We estimate by 2009, 61% of former reservoir sediments were removed during dam removal and channel reconstruction. Between 2009 and 2011 renewed sedimentation within the former reservoir represented approximately 7.85% of the original reservoir volume. The HEC‐RAS IV models show the largest impacts of the dam and riffle structure occur at flood magnitudes at or below bankfull. Thus, the riffle and the dam similarly alter channel hydraulics and sediment transport. As such, our models indicate that the La Valle Dam project was a dam replacement rather than a removal. Our results confirm that channel reconstruction method can alter channel hydraulics, geomorphology, and sediment mobility.     

VEGETATION INFLUENCE ON SMALL STREAM SILTATION1

ABSTRACT: A meandering stream channel was simulated in the Hydraulics Laboratory at Colorado State University and a series of tests was conducted using four types of vegetation to evaluate the potential effects of vegetation on sediment deposition and retention in a stream channel. The data collected included average flow velocity, flow depth, length of vegetation, density of vegetation, cross-sectional area of the vegetative stem, wetted perimeter of the vegetative stem, and injection and flushing time. The findings indicated that the vegetation could retain from 30 to 70 percent of the deposited sediments. The ability of vegetation to entrap and retain sediment is related to the length and cross-sectional area of the vegetation. The variables describing the flow and the vegetative properties were combined to form a predictive parameter, the sedimentation factor (S_d) that can be compared with the amount of sediment entrapped by vegetation in a stream system. A relation was developed correlating vegetation length to sediment retention after flushing for flexibility and rigid vegetation.     

OPTIMAL CONTROL OF RESERVOIR RELEASES TO MINIMIZE SEDIMENTATION IN RIVERS AND RESERVOIRS1

ABSTRACT: An optimal control methodology and computational model are developed to evaluate multi‐reservoir release schedules that minimize sediment scour and deposition in rivers and reservoirs. The sedimentation problem is formulated within a discrete‐time optimal control framework in which reservoir releases represent control variables and reservoir bed elevations, storage levels, and river bed elevations represent state variables. Constraints imposed on reservoir storage levels and releases are accommodated using a penalty function method. The optimal control model consists of two interfaced components: a one‐dimensional finite‐difference simulation module used to evaluate flow hydraulics and sediment transport dynamics, and a successive approximation linear quadratic regulator (SALQR) optimization algorithm used to update reservoir release policies and solve the augmented control problem. Hypothetical two‐reservoir and five‐reservoir networks are used to demonstrate the methodology and its capabilities, which is a vital phase towards the development of a more robust optimal control model and application to an existing multiple‐reservoir river network.     

Five-wheel framework for system-based monitoring of heavy metal pollution in rivers

Sectorial approach for monitoring heavy metal pollution in rivers has failed to report realistic pollution status and associated ecological and human health risks. The increasing spread of heavy metals from different sources and emerging risks to human and environmental health call for reexamining heavy metal pollution monitoring frameworks. Also, the sources, spread, and load of heavy metals in the environment have changed significantly over time, requiring consequent modification in the monitoring frameworks. Therefore, studies on heavy metal monitoring in rivers conducted in the last decade were evaluated for experimental designs, research frameworks, and data presentations. Most studies (∼99%) (i) lacked inclusiveness of all environmental compartments; (ii) focused on “one pollutant – one/two compartment” or sometimes “one pollutant – one compartment – one effect” approach; and (iii) remained “data-rich but information poor.” An ecological approach with integrative system thinking is proposed to develop a holistic approach for monitoring river pollution. It is visualized that heavy metal monitoring, risk analyses, and water management must incorporate tracking pollutants in different environmental compartments of a river (water, sediment, and floodplain/bank soil) and consider correlating it with riverbank land use. The systems-based pollution monitoring and assessment studies will reveal the critical factors that drive heavy metals pollutant movement in ecosystems and associated potential risks to the environment, wildlife, and humans. Also, water quality and pollution indexing tools would help better communicate complex pollution data and associated risks among all stakeholders. Therefore, integrating systems approaches in scientific- and policy-based tools would help sustainably manage the health of rivers, wildlife, and humans.     

Effect of Human Activities on Overall Trend of Sedimentation in the Lower Yellow River,China

The Yellow River has been intensively affected by human activities, particularly in the past 50 years, including soil–water conservation in the upper and middle drainage basin, flood protection in the lower reaches, and flow regulation and water diversion in the whole drainage basin. All these changes may impact sedimentation process of the lower Yellow River in different ways. Assessing these impacts comprehensively is important for more effective environmental management of the drainage basin. Based on the data of annual river flow, sediment load, and channel sedimentation in the lower Yellow River between 1950 and 1997, the purpose of this paper is to analyze the overall trend of channel sedimentation rate at a time scale of 50 years, and its formative cause. It was found in this study that erosion control measures and water diversion have counteractive impacts on sedimentation rate in the lower Yellow River. Although both annual river flow and sediment decreased, there was no change in channel sedimentation rate. A regression analysis indicated that the sedimentation in the lower Yellow River decreased with the sediment input to the lower Yellow River but increased with the river flow input. In the past 30–40 years, the basin-wide practice of erosion and sediment control measures resulted in a decline in sediment supply to the Yellow River; at the same time, the human development of water resources that required river flow regulation and water diversion caused great reduction in river flow. The former may reduce the sedimentation in the lower Yellow River, but the reduction of river flow increased the sedimentation. When their effects counterbalanced each other, the overall trend of channel sedimentation in the lower Yellow River remained unchanged. This fact may help us to better understand the positive and negative effects of human activities in the Yellow River basin and to pay more attention to the negative effect of the development of water resources. The results of this study demonstrate that, if the overuse of river water cannot be controlled, the reduction of channel sedimentation in the lower Yellow River cannot be realized through the practice of erosion and sediment control measures.     

The Effect of Channelization on Floodplain Sediment Deposition and Subsidence Along the Pocomoke River,Maryland1

Kroes, Daniel E. and Cliff R. Hupp, 2010. The Effect of Channelization on Floodplain Sediment Deposition and Subsidence Along the Pocomoke River, Maryland. Journal of the American Water Resources Association (JAWRA) 46(4): 686-699. DOI: 10.1111/j.1752-1688.2010.00440.x Abstract: The nontidal Pocomoke River was intensively ditched and channelized by the mid-1900s. In response to channelization; channel incision, head-cut erosion, and spoil bank perforation have occurred in this previously nonalluvial system. Six sites were selected for study of floodplain sediment dynamics in relation to channel condition. Short- and long-term sediment deposition/subsidence rates and composition were determined. Short-term rates (four years) ranged from 0.6 to 3.6 mm/year. Long-term rates (15-100+ years) ranged from −11.9 to 1.7 mm/year. ¹³⁷Cs rates (43 years) indicate rates of 0.24 to 7.4 mm/year depending on channel condition. Channelization has limited contact between streamflow and the floodplain, resulting in little or no sediment retention in channelized reaches. Along unchannelized reaches, extended contact and depth of river water on the floodplain resulted in high deposition rates. Drainage of floodplains exposed organic sediments to oxygen resulting in subsidence and releasing stored carbon. Channelization increased sediment deposition in downstream reaches relative to the presettlement system. The sediment storage function of this river has been dramatically altered by channelization. Results indicate that perforation of spoil banks along channelized reaches may help to alleviate some of these issues.     

Sourcing sediment using multiple tracers in the catchment of Lake Argyle,Northwestern Australia

Control of sedimentation in large reservoirs requires soil conservation at the catchment scale. In large, heterogeneous catchments, soil conservation planning needs to be based on sound information, and set within the framework of a sediment budget to ensure that all of the potentially significant sources and sinks are considered. The major sources of sediment reaching the reservoir, Lake Argyle, in tropical northwestern Australia, have been determined by combining measured sediment fluxes in rivers with spatial tracer-based estimates of proportional contributions from tributaries of the main stream entering the lake, the Ord River. The spatial tracers used are mineral particle magnetics, the strontium isotopic ratio, and the neodymium isotopic ratio. Fallout of ¹³⁷Cs has been used to estimate the proportion of the sediment in Lake Argyle eroded from surface soils by sheet and rill erosion, and, by difference, the proportion eroded from subsurface soils by gully and channel erosion. About 96% of the sediment in the reservoir has come from less than 10% of the catchment, in the area of highly erodible soils formed on Cambrian-age sedimentary rocks. About 80% of the sediment in the reservoir has come from gully and channel erosion. A major catchment revegetation program, designed to slow sedimentation in the reservoir, appears to have had little effect because it did not target gullies, the major source of sediment. Had knowledge of the sediment budget been available before the revegetation program was designed, an entirely different approach would have been taken.     

Legacy Effects of Colonial Millponds on Floodplain Sedimentation,Bank Erosion,and Channel Morphology,Mid‐Atlantic,USA1

Abstract: Many rivers and streams of the Mid‐Atlantic Region, United States (U.S.) have been altered by postcolonial floodplain sedimentation (legacy sediment) associated with numerous milldams. Little Conestoga Creek, Pennsylvania, a tributary to the Susquehanna River and the Chesapeake Bay, is one of these streams. Floodplain sedimentation rates, bank erosion rates, and channel morphology were measured annually during 2004‐2007 at five sites along a 28‐km length of Little Conestoga Creek with nine colonial era milldams (one dam was still in place in 2007). This study was part of a larger cooperative effort to quantify floodplain sedimentation, bank erosion, and channel morphology in a high sediment yielding region of the Chesapeake Bay watershed. Data from the five sites were used to estimate the annual volume and mass of sediment stored on the floodplain and eroded from the banks for 14 segments along the 28‐km length of creek. A bank and floodplain reach based sediment budget (sediment budget) was constructed for the 28 km by summing the net volume of sediment deposited and eroded from each segment. Mean floodplain sedimentation rates for Little Conestoga Creek were variable, with erosion at one upstream site (?5 mm/year) to deposition at the other four sites (highest = 11 mm/year) despite over a meter of floodplain aggradation from postcolonial sedimentation. Mean bank erosion rates range between 29 and 163 mm/year among the five sites. Bank height increased 1 m for every 10.6 m of channel width, from upstream to downstream (R² = 0.79, p < 0.0001) resulting in progressively lowered hydraulic connectivity between the channel and the floodplain. Floodplain sedimentation and bank erosion rates also appear to be affected by the proximity of the segments to one existing milldam, which promotes deposition upstream and scouring downstream. The floodplain and bank along the 28‐km reach produced a net mean sediment loss of 5,634 Mg/year for 2004‐2007, indicating that bank erosion was exceeding floodplain sedimentation. In particular, the three segments between the existing dam and the confluence with the Conestoga River (32% of the studied reach) account for 97% of the measured net sediment budget. Future research directed at understanding channel equilibria should facilitate efforts to reduce the sediment impacts of dam removal and legacy sediment.     

Alluvial Sedimentation and Erosion in an Urbanizing Watershed,Gwynns Falls,Maryland1

Abstract: Earlier measurements of stream channel geometry on 19 reaches were repeated to provide a longitudinal study of stream channel adjustment over 13 years (1987‐2000) in the urbanizing Gwynns Falls, Maryland watershed. We observed both enlargement and reduction in channel size, depending on the extent of upstream development, the timing and location of urbanization and upstream channel adjustment, and the presence of hydrologic constrictions and grade controls. Based on a relatively simple visual assessment of the composition, size, and extent of instream sediment storage, we categorized stream reaches into three phases: aggraded (7 sites), early erosion (7 sites), and late erosion (5 sites). Aggraded sites had point and lateral bars mantled with fine‐grained sediment and experienced some reduction in cross‐sectional area, primarily through the deposition of fine‐grained material on bars in the channel margins. Early erosion sites had smaller bars and increases in channel cross‐sectional area as a consequence of the evacuation of in‐channel fine‐grained sediment. Fine‐grained sediments were either entirely absent or found only at a few high bar elevations at late erosion sites. Sediment evacuation from late erosion sites has both enlarged and simplified channels, as demonstrated by an increase in cross‐sectional area and a strong decrease in channel width variation. Channel cross‐sectional area enlargement, reduced channel width variation, and channel incision were ubiquitous at erosion sites. As a result, overbank flows were less common in the erosion sites as determined by high water marks left by a 2‐year flood that occurred during the study period. Principal causes for channel changes appear to be increased high flow durations and reduced sediment supply. Spatial variation in channel conditions could not be tied simply to sub‐basin impervious cover or watershed area. In‐channel sediment storage is a useful indicator of channel form and adjustment. When combined with information on development and sedimentation conditions in the contributing drainage, instream sediment storage can be used to effectively assess future channel adjustments.