Allen C. Gellis

Characteristic length scales and time‐averaged transport velocities of suspended sediment in the mid‐Atlantic Region, USA

[1] Watershed Best Management Practices (BMPs) are often designed to reduce loading from particle-borne contaminants, but the temporal lag between BMP implementation and improvement in receiving water quality is difficult to assess because particles are only moved downstream episodically, resting for long periods in storage between transport events. A theory is developed that describes the downstream movement of suspended sediment particles accounting for the time particles spend in storage given sediment budget data (by grain size fraction) and information on particle transit times through storage reservoirs. The theory is used to define a suspended sediment transport length scale that describes how far particles are carried during transport events, and to estimate a downstream particle velocity that includes time spent in storage. At 5 upland watersheds of the mid-Atlantic region, transport length scales for silt-clay range from 4 to 60 km, while those for sand range from 0.4 to 113 km. Mean sediment velocities for silt-clay range from 0.0072 km/yr to 0.12 km/yr, while those for sand range from 0.0008 km/yr to 0.20 km/yr, 4–6 orders of magnitude slower than the velocity of water in the channel. These results suggest lag times of 100–1000 years between BMP implementation and effectiveness in receiving waters such as the Chesapeake Bay (where BMPs are located upstream of the characteristic transport length scale). Many particles likely travel much faster than these average values, so further research is needed to determine the complete distribution of suspended sediment velocities in real watersheds.

Sediment source fingerprinting as an aid to catchment management: A review of the current state of knowledge and a methodological decision-tree for end-users

The growing awareness of the environmental significance of fine-grained sediment fluxes through catchment systems continues to underscore the need for reliable information on the principal sources of this material. Source estimates are difficult to obtain using traditional monitoring techniques, but sediment source fingerprinting or tracing procedures, have emerged as a potentially valuable alternative. Despite the rapidly increasing numbers of studies reporting the use of sediment source fingerprinting, several key challenges and uncertainties continue to hamper consensus among the international scientific community on key components of the existing methodological procedures. Accordingly, this contribution reviews and presents recent developments for several key aspects of fingerprinting, namely: sediment source classification, catchment source and target sediment sampling, tracer selection, grain size issues, tracer conservatism, source apportionment modelling, and assessment of source predictions using artificial mixtures. Finally, a decision-tree representing the current state of knowledge is presented, to guide end-users in applying the fingerprinting approach.

Channel evolution and hydrologic variations in the Colorado River basin: Factors influencing sediment and salt loads

Abstract Suspended-sediment and dissolved-solid (salt) loads decreased after the early 1940s in the Colorado Plateau portion of the Colorado River basin, although discharge of major rivers — the Colorado, Green and San Juan — did not change significantly. This decline followed a period of high sediment yield caused by arroyo cutting. Reduced sediment loads have previously been explained by a change in sediment sampling procedures or changes in climate, land-use and conservation practices. More recent work has revealed that both decreased sediment production and sediment storage in channels of tributary basins produced the decline of sediment and salt loads. Sediment production and sediment storage are important components of incised-channel evolution, which involves sequential channel deepening, widening and finally floodplain formation. Accordingly, the widespread arroyo incision of the late nineteenth century resulted initially in high sediment loads. Since then, loads have decreased as incised channels (arroyos) have stabilized and begun to aggrade. However, during the 1940s, a period of low peak discharges permitted vegetational colonization of the valley floors, which further reduced sediment loads and promoted channel stabilization. This explanation is supported by experimental studies and field observations. Both geomorphic and hydrologic factors contributed to sediment storage and decreased sediment and salt loads in the upper Colorado River basin.

Land-Use Effects on Erosion, Sediment Yields, and Reservoir Sedimentation: A Case Study in the Lago Loíza Basin, Puerto Rico

Lago Loíza impounded in 1953 to supply San Juan, Puerto Rico, with drinking water; by 1994, it had lost 47% of its capacity. To characterize sedimentation in Lago Loíza, a study combining land-use history, hillslope erosion rates, and subbasin sediment yields was conducted. Sedimentation rates during the early part of the reservoirs operation (1953-1963) were slightly higher than the rates during 1964-1990. In the early history of the reservoir, cropland comprised 48% of the basin and erosion rates were high. Following economic shifts during the 1960s, cropland was abandoned and replaced by forest, which increased from 7.6% in 1950 to 20.6% in 1987. These land-use changes follow a pattern similar to the northeastern United States. Population in the Lago Loíza Basin increased 77% from 1950 to 1990, and housing units increased 194%. Sheetwash erosion measured from 1991 to 1993 showed construction sites had the highest sediment concentration (61,400 ppm), followed by cropland (47,400 ppm), pasture (3510 ppm), and forest (2050 ppm). This study illustrates how a variety of tools and approaches can be used to understand the complex interaction between land use, upland erosion, fluvial sediment transport and storage, and reservoir sedimentation.

Developing a geomorphic approach for ranking watersheds for rehabilitation, Zuni Indian Reservation, New Mexico

Abstract As a result of past erosion problems on the Zuni Indian Reservation in western New Mexico, the US Congress in 1990 authorized the Zuni Tribe to begin a program for watershed rehabilitation. This paper describes an approach to rank the most appropriate watersheds for rehabilitation for the Zuni Reservation. The approach was based on data collected during a 3-year study on geomorphic and anthropogenic characteristics of the Rio Nutria Watershed, including data on (i) arroyo cross-sectional changes, (ii) erosion-control structures, and (iii) sheetwash erosion. Results of this 3-year study indicated that 61 of 85 channel cross-sections aggraded and channels with lower width-to-depth ratios eroded. Results on assessment of erosion-control structures, some dating back to the 1930s, indicated that 60% of earthen dams and 22% of rock-and-brush structures were breached or flanked in the Rio Nutria Watershed. Sheetwash erosion measured on five land-cover sites (sagebrush, pasture, chained pinon and juniper, unchained pinon and juniper, and ponderosa pine) indicated chained pinon and juniper sites and pasture sites had the highest volume-weighted sediment concentrations of 13,000 and 9970 ppm, respectively. Based on interpretations of the 3-year study in the Rio Nutria Watershed, a two-stage approach was developed to rank the most appropriate watersheds for rehabilitation on the Zuni Reservation. In the first stage, the reservation was divided into eight major watersheds, which were ranked according to the most potential for erosion. In the second stage, the watershed with the most potential for erosion was divided into sub-basins, which were ranked according to the most potential for erosion. Quantitative and qualitative information on physical and anthropogenic factors were used at each stage to rank the watersheds. Quantitative physical data included headcut density, percentage of bare ground, percentage of chained area, channel width-to-depth ratio, change in channel density from 1934 to 1988, and sheetwash erosion rates. Qualitative physical data included erosion rankings on the main channels, tributaries, and entire basins. Anthropogenic data included density of dirt roads and condition of erosion-control structures. A community survey and agricultural acreage were also used in the selection process. The first stage analysis resulted in the selection of the Rio Nutria Watershed as the most appropriate major watershed for rehabilitation. In the second stage, the Rio Nutria Watershed was divided into 15 sub-basins; the analysis indicated the highest priority sub-basins for rehabilitation were Benny Draw, Coal Mine Canyon Draw, and Garcia Draw. Many erosion-control projects that have been conducted in the southwestern United States since the 1930s lack documentation on the approach used to select treatment areas. This study demonstrates that by using geomorphic data and anthropogenic factors a logical approach to ranking watersheds for rehabilitation can be developed.

Watershed sediment source identification: tools, approaches, and case studies

Many of us who have watched a stream turn turbid during a rainstorm have often wondered—where is all that sediment coming from? Geomorphologists, engineers, and environmental scientists have thought about this question for decades and have placed understanding sediment sources under the general framework of sediment budgets. This framework which examines not only sediment sources but the storage, transport, and delivery of sediment has been used to understand surficial processes, as well as address management concerns as they relate to stream turbidity and surface-water quality. Traditional tools used in sediment budgets have included field measurements of erosion, storage and transport rates, photogrammetric analysis, and modeling. The relationship between soil erosion and sediment yield at the watershed outlet has been long identified as a major research need (Walling 1983) and is still a poorly understood or misunderstood component of fluvial sediment transport (Kinnell 2004). In recent decades, sediment fingerprinting approaches using the geochemical and physical properties of sediment to determine sediment sources have become increasingly popular. This special issue on Watershed Sediment Source Identification, for the Journal of Soils and Sediments was produced as a result of a special session convened at the Association of American Geographers (AAG) 2012 annual meeting in New York City, NY, USA. The session was entitled “Watershed Sediment Source Identification” and brought together leading experts in the field of sediment sourcing. As a result of the AAG session, the Journal of Soils and Sediments offered to have the session and a few other papers on the same topic published as a special issue in the journal. This special issue reflects contributions from 29 researchers discussing sediment sourcing results from studies in Africa, Asia, Europe, and North America where settings range from urban to agricultural to forested. Sediment source assessment is not only important to our understanding of sediment dynamics in fluvial systems but is increasingly becoming an important management tool. The papers in this special issue reflect a range of studies that address both topics. Studies emphasize traditional field approaches to determine sediment yields and sources (Davis and Sims 2013; Zhu 2013) as well as the use of the sediment fingerprinting approach to determine significant sediment sources (Dutton et al. 2013; Gellis and Noe 2013; Huisman et al. 2013; Koiter et al. 2013; Mckinley et al. 2013; Voli et al. 2013). Several papers highlight site-specific methods and approaches to streamline the sediment fingerprinting approach into a practical management tool. Other papers relate findings from sediment fingerprinting to hydrologic conditions, and caution users on over-interpreting the sediment fingerprinting results. Walling (2013), in the opening paper, provides a history on the sediment fingerprinting approach and illustrates how publications on the subject have increased exponentially since the 1970s. A review of these publications indicate that many studies have used sediment fingerprinting to determine sediment sources as a means to improve our understanding of erosion and sediment delivery processes. Other papers use sediment fingerprinting as a management tool. Walling (2013) lists key advances in the sediment fingerprinting approach over the last 30 years that include the use of multiple or composite fingerprints, statistical tests and models, size and organic correction factors, increased source and target assessments, and improved estimates of uncertainty. Given these advances in the sediment A. C. Gellis (*) U.S. Geological Survey, 5522 Research Park Drive, Baltimore, MD 21228, USA e-mail: agellis@usgs.gov

Erosion, storage, and transport of sediment in two subbasins of the Rio Puerco, New Mexico

Arroyos in the American Southwest proceed through cut-and-fill cycles that operate at centennial to millennial time scales. The geomorphic community has put much effort into understanding the causes of arroyo cutting in the late Quaternary and in the modern record (late 1800s), while little effort has gone into understanding how arroyos fill and the sources of this fill. Here, we successfully develop a geographic information system (GIS)–modeled sediment budget that is based on detailed field measurements of hillslope and channel erosion and deposition. Field measurements were made in two arroyo basins draining different lithologies and undergoing different land disturbance (Volcano Hill Wash, 9.30 km 2 ; Arroyo Chavez, 2.11 km 2 ) over a 3 yr period. Both basins have incised channels that formed in response to the late nineteenth-century incision of the Rio Puerco. Large volumes of sediment were generated during arroyo incision, equal to more than 100 yr of the current annual total sediment load (bed load + suspended load) in each basin. Downstream reaches in both arroyos are presently aggrading, and the main source of the sediment is from channel erosion in upstream reaches and first- and second-order tributaries. The sediment budget shows that channel erosion is the largest source of sediment in the current stage of the arroyo cycle: 98% and 80% of the sediment exported out of Volcano Hill Wash and Arroyo Chavez, respectively. The geomorphic surface most affected by arroyo incision and one of the most important sediment sources is the valley alluvium, where channel erosion, gullying, soil piping, and grazing all occur. Erosion rates calculated for the entire Volcano Hill Wash (–0.26 mm/yr) and Arroyo Chavez (–0.53 mm/yr) basins are higher than the modeled upland erosion rates in each basin, reflecting the large contributions from channel erosion. Erosion rates in each basin are affected by a combination of land disturbance (grazing) and lithology—erodible sandstones and shales in Arroyo Chavez compared with basalt for Volcano Hill Wash. Despite these differences, hillslope sediment yields are similar to long-term denudation rates. As the arroyo fills over time from mouth to headwaters, hillslope sediment becomes a more significant sediment source.

Channel response to sediment release: insights from a paired analysis of dam removal

Dam removals with unmanaged sediment releases are good opportunities to learn about channel response to abruptly increased bed material supply. Understanding these events is important because they affect aquatic habitats and human uses of floodplains. A longstanding paradigm in geomorphology holds that response rates to landscape disturbance exponentially decay through time. However, a previous study of the Merrimack Village Dam (MVD) removal on the Souhegan River in New Hampshire, USA, showed that an exponential function poorly described the early geomorphic response. Erosion of impounded sediments there was two-phased. We had an opportunity to quantitatively test the two-phase response model proposed for MVD by extending the record there and comparing it with data from the Simkins Dam removal on the Patapsco River in Maryland, USA. The watershed sizes are the same order of magnitude (102 km2), and at both sites low-head dams were removed (~3–4 m) and ~65 000 m3 of sand-sized sediments were discharged to low-gradient reaches. Analyzing four years of repeat morphometry and sediment surveys at the Simkins site, as well as continuous discharge and turbidity data, we observed the two-phase erosion response described for MVD. In the early phase, approximately 50% of the impounded sediment at Simkins was eroded rapidly during modest flows. After incision to base level and widening, a second phase began when further erosion depended on floods large enough to go over bank and access impounded sediments more distant from the newly-formed channel. Fitting functional forms to the data for both sites, we found that two-phase exponential models with changing decay constants fit the erosion data better than single-phase models. Valley width influences the two-phase erosion responses upstream, but downstream responses appear more closely related to local gradient, sediment re-supply from the upstream impoundments, and base flows. Copyright

Stream Sediment Sources in Midwest Agricultural Basins with Land Retirement along Channel

Documenting the effects of agricultural land retirement on stream-sediment sources is critical to identifying management practices that improve water quality and aquatic habitat. Particularly difficult to quantify are the effects from conservation easements that commonly are discontinuous along channelized streams and ditches throughout the agricultural midwestern United States. Our hypotheses were that sediment from cropland, retired land, stream banks, and roads would be discernible using isotopic and elemental concentrations and that source contributions would vary with land retirement distribution along tributaries of West Fork Beaver Creek in Minnesota. Channel-bed and suspended sediment were sampled at nine locations and compared with local source samples by using linear discriminant analysis and a four-source mixing model that evaluated seven tracers: In, P, total C, Be, Tl, Th, and Ti. The proportion of sediment sources differed significantly between suspended and channel-bed sediment. Retired land contributed to channel-bed sediment but was not discernible as a source of suspended sediment, suggesting that retired-land material was not mobilized during high-flow conditions. Stream banks were a large contributor to suspended sediment; however, the percentage of stream-bank sediment in the channel bed was lower in basins with more continuous retired land along the riparian corridor. Cropland sediments had the highest P concentrations; basins with the highest cropland-sediment contributions also had the highest P concentrations. Along stream reaches with retired land, there was a lower proportion of cropland material in suspended sediment relative to sites that had almost no land retirement, indicating less movement of nutrients and sediment from cropland to the channel as a result of land retirement.

Sediment source fingerprinting for informing catchment management: Methodological approaches, problems and uncertainty

The application of the sediment source fingerprinting approach has accelerated rapidly in recent years (Davis and Fox, 2009; Krishnappan et al., 2009; Mukundan et al., 2012; Walling, 2013; Gellis and Mukundan, 2013; Walling and Foster, 2016; Walling and Collins, 2016). One key driver for this increasing interest concerns the need to manage the excess fine (typically <63 mm fraction) sediment loads delivered to rivers worldwide and their associated detrimental impacts, including amongst others, those on water quality, treatability, and aquatic ecology (Gellis and Walling, 2011). The cost-effective targeting of management strategies requires reliable information on the key sources of the sediment problem at landscape or catchment scales, and sediment source tracing has the potential to avoid some of the logistical and cost constraints associated with traditional procedures for investigating erosion processes and subsequent sediment delivery to river channels. Since the pioneering work of Klages and Hsieh (1975) and Wall and Wilding (1976), the number and diversity of studies applying sediment source fingerprinting has accelerated to the present, but therein, lies an urgent need for the scientific community to address the lack of standard procedures and protocols for the various key stages comprising the overall approach. From a conceptual stance, the fingerprinting approach is based on a small number of key assumptions, including: the key potential sources of sediment in any landscape or catchment can be readily identified for informing targeted sampling campaigns; representative source and sediment samples when compared using their properties (e.g. geochemistry, mineral-magnetism) or composite signatures (multiple individual properties; sensu Walling et al., 1993) provides a reliable basis for apportioning the key sources involved, and; tracer properties remain relatively conservative during transport from source to sink. Despite these underpinning elementary assumptions, much diversity has evolved in the procedures used to apply the general approach. Much of the current research and publications on sediment fingerprinting have objectives that include but are not limited to testing: (1) a growing number of different types of potential fingerprint properties, (2) contrasting approaches to classifying potential sediment source areas, (3) robust source discrimination using a growing variety of statistical tests (4) variations in numerical mixing model structures in response to issues associated with particle size and organic matter enrichment or depletion, (5) the direct comparability of source and sediment samples, and (6) the need to be explicit about the uncertainties associated with the source predictions. At the same time, studies applying the source