Patrick Belmont

Large Shift in Source of Fine Sediment in the Upper Mississippi River

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Automatic geomorphic feature extraction from lidar in flat and engineered landscapes

[1] High-resolution topographic data derived from light detection and ranging (lidar) technology enables detailed geomorphic observations to be made on spatially extensive areas in a way that was previously not possible. Availability of this data provides new opportunities to study the spatial organization of landscapes and channel network features, increase the accuracy of environmental transport models, and inform decisions for targeting conservation practices. However, with the opportunity of increased resolution topographic data come formidable challenges in terms of automatic geomorphic feature extraction, analysis, and interpretation. Low-relief landscapes are particularly challenging because topographic gradients are low, and in many places both the landscape and the channel network have been heavily modified by humans. This is especially true for agricultural landscapes, which dominate the midwestern United States. The goal of this work is to address several issues related to feature extraction in flat lands by using GeoNet, a recently developed method based on nonlinear multiscale filtering and geodesic optimization for automatic extraction of geomorphic features (channel heads and channel networks) from high-resolution topographic data. Here we test the ability of GeoNet to extract channel networks in flat and human-impacted landscapes using 3 m lidar data for the Le Sueur River Basin, a 2880 km 2 subbasin of the Minnesota River Basin. We propose a curvature analysis to differentiate between channels and manmade structures that are not part of the river network, such as roads and bridges. We document that Laplacian curvature more effectively distinguishes channels in flat, human-impacted landscapes compared with geometric curvature. In addition, we develop a method for performing automated channel morphometric analysis including extraction of cross sections, detection of bank locations, and identification of geomorphic bankfull water surface elevation. Using the slope plotted along each channel-floodplain cross section, we demonstrate the ability to identify and measure the height of river banks and bluffs. Finally, we present an example that demonstrates how extracting such features automatically is important for modeling channel evolution, water and sediment transport, and channel-floodplain sediment exchange.

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

Toward generalizable sediment fingerprinting with tracers that are conservative and nonconservative over sediment routing timescales

PurposeThe science of sediment fingerprinting has been evolving rapidly over the past decade and is well poised to improve our understanding, not only of sediment sources, but also the routing of sediment through watersheds. Here, we discuss channel–floodplain processes that may convolute or modify the sediment fingerprinting signature of alluvial bank/floodplain sources and explore the use of nonconservative tracers for differentiating sediment derived from surface soil erosion from that of near-channel fluvial erosion.Materials and methodsWe use a mathematical model to demonstrate the theoretical effects of channel–floodplain exchange on conservative and nonconservative tracers. Then, we present flow, sediment gauging data, and geochemical measurements of long- (meteoric beryllium-10, 10Be) and short-lived (excess lead-210 and cesium-137, 210Pbex and 137Cs, respectively) radionuclide tracers from two study locations: one above, and the other below, a rapidly incising knick zone within the Maple River watershed, southern Minnesota.Results and discussionWe demonstrate that measurements of 10Be, 210Pbex, and 137Cs associated with suspended sediment can be used to distinguish between the three primary sediment sources (agricultural uplands, bluffs, and banks) and estimate channel–floodplain exchange. We observe how the sediment sources systematically vary by location and change over the course of a single storm hydrograph. While sediment dynamics for any given event are not necessarily indicative of longer-term trends, the results are consistent with our geomorphic understanding of the system and longer-term observations of sediment dynamics. We advocate for future sediment fingerprinting studies to develop a geomorphic rationale to explain the distribution of the fingerprinting properties for any given study area, with the intent of developing a more generalizable, process-based fingerprinting approach.ConclusionsWe show that measurements of conservative and nonconservative tracers (e.g., long- and short-lived radionuclides) can provide spatially integrated, yet temporally discrete, insights to constrain sediment sources and channel–floodplain exchange at the river network-scale. Fingerprinting that utilizes nonconservative tracers requires that the nonconservative behavior is predictable and verifiable.

A numerical model to develop long-term sediment budgets using isotopic sediment fingerprints

Developing accurate long-term, basin-scale sediment budgets using isotopic sediment fingerprints requires a sediment routing model that not only accounts for a range of sediment source terms (e.g. tributaries, surface erosion and erosion of bluffs and terraces) but also considers the variation in time of volume and tracer concentration for the sediment stored in the floodplain. This is accomplished here using a tracer routing model that accounts for production and decay of radioisotopes in the floodplain. The numerical model focuses on the average (i.e. across many hydrographs or years) budget of sediment and tracers at reach scale. To account for storage and remobilization of bulk sediment and/or tracer material, the model represents the floodplain as a system that can gain or lose mass depending on overbank deposition and net bank erosion rates. Isotopic tracers within the floodplain reservoir can be produced as a function of cosmic ray bombardment or atmospheric fallout, and can decay according to a first-order rate equation. Governing equations are derived using a simplified geometry that treats rivers at reach scale: channel sinuosity and migration rates are user-specified parameters, exchange of sediment and tracers between the river and floodplain is modeled at each cross section, and governing equations are derived in a 1D, width-averaged formulation. When the system reaches mobile equilibrium, the sediment deposited on the floodplain through overbank deposition is balanced by the sediment eroded from the floodplain through channel migration and by sediment contributed from external sources. The model is applied to a generic river system and is shown to converge over time to an equilibrium condition that is consistent with an independent analytical solution.

Closing the Gap Between Watershed Modeling, Sediment Budgeting, and Stream Restoration

Stream Restorat Approaches, Anal Geophysical Mon Copyright 2011 b 10.1029/2011GM The connection between stream restoration and sediment budgeting runs both ways: stream restoration is proposed as a means to reduce sediment yields, but an accurate understanding of sediment supply is necessary to design an effective project. Recent advances in monitoring technology, geochemical techniques, high-resolution topography data, and numerical modeling provide new opportunities to estimate sediment erosion, transport, and deposition rates; upscale them in a geomorphically relevant fashion; and synthesize sediment dynamics at watershed scales. For practical application at large scale, watershed models used to predict yield often do not resolve lower-order channels, leaving an essential “blind spot” regarding sediment processes. We illustrate the challenges and emerging approaches for estimating sediment budgets using examples from two very different physiographic settings: the Mid-Atlantic Piedmont and the agricultural plains of southern Minnesota. We highlight common challenges and themes in defining an effective watershed sediment model. In both cases, reliable estimates of sediment yield depend essentially on the accurate identification of sediment sources and sinks and, hence, require careful delineation of landscape units and identification of dominant sediment sources and sinks. The primary elements needed to bridge the gap between sediment budgeting, watershed modeling, and stream restoration are (1) specificity regarding location, mechanism, and rates of erosion, (2) accurate accounting of sediment storage, (3) appropriate methods for upscaling local observations, (4) efficient means for incorporating multiple lines of evidence to constrain budget estimates, and (5) stream restoration methods that incorporate sediment supply in assessment and design procedures.

Landscape Evolution in South-Central Minnesota and the Role of Geomorphic History on Modern Erosional Processes

The Minnesota River Valley was carved during catastrophic drainage of glacial Lake Agassiz at the end of the late Pleistocene. The ensuing base-level drop on tributaries created knickpoints that excavated deep valleys as they migrated upstream. A sediment budget compiled in one of these tributaries, the Le Sueur River, shows that these deep valleys are now the primary source of sediment to the Minnesota River. To compare modern sediment loads with pre-European settlement erosion rates, we analyzed incision history using fluvial terrace ages to constrain a valley incision model. Results indicate that even though GSA Today, v. 21. no. 9, doi: 10.1130/G121A.1.

Identifying Sediment Sources and Sinks in the Root River, Southeastern Minnesota

Excessive loading of fine sediment is a prominent cause of river impairment, not only due to direct effects on biota and habitat but because sediment is often laden with excess nutrients, metals, and toxic substances. Determining the sources and transport pathways of sediment has proven challenging. The Root River watershed in southeastern Minnesota was listed under section 303d of the U.S. Clean Water Act as having forty-three impaired reaches, raising these questions: Where is the fine sediment coming from? What proportions of the sediment are from uplands versus near-channel erosion? How much of the excess sediment loading is caused by modern land use and water management versus the legacy of past land use? Managing fine sediment at the watershed scale requires that we identify potential sources and sinks throughout the watershed, measure source contributions, and understand transport pathways of fine sediment. Here we utilize sediment fingerprinting techniques involving long- and short-lived radionuclide tracers, specifically beryllium-10 (10Be), excess lead-210 (210Pbex), and cesium-137 (137Cs), in combination with other supporting data sets to address the preceding questions. We document a shift in hydrologic regime and that sediment fluxes are sensitive to both magnitude and sequence of flood events. Geomorphic analysis indicates that many river reaches have accessible near-channel sources that contribute the dominant proportion of the washload flux in subwatersheds. Lastly, geochemical tracer analyses of floodplains and hillslope soils indicate that historic erosion has been variable across the landscape and the majority of suspended sediment in the river today is sourced from floodplains and terraces.

Solving water quality problems in agricultural landscapes: New approaches for these nonlinear, multiprocess, multiscale systems

Changes in climate and agricultural practices are putting pressure on agro-environmental systems all over the world. Predicting the effects of future management or conservation actions has proven exceptionally challenging in these complex landscapes. We present a perspective, gained from a decade of research and stakeholder involvement in the Minnesota River Basin, where research findings have influenced solutions and policy in directions not obvious at the outset. Our approach has focused on identifying places, times, and processes of significant change and developing reduced complexity predictive frameworks that can inform mitigation actions.

Comment on “Climate and agricultural land use change impacts on streamflow in the upper midwestern United States” by Satish C. Gupta et al.

The paper “Climate and agricultural land use change impacts on streamflow in the upper midwestern United States” by Satish C. Gupta, Andrew C. Kessler, Melinda K. Brown, and Francis Zvomuya (hereafter referred to as Gupta et al.) purports to evaluate “the relative importance of changes in precipitation and LULC (land use, land cover) on streamflow in 29 Hydrologic Unit Code 008 watersheds in the Upper Midwestern United States.” However, as we report here, the approach used by Gupta et al. is wholly inadequate for making such an evaluation. Gupta et al. use strong language to criticize other studies and imply a level of certainty that goes well beyond, and in some cases is entirely unsupported by, the results they have presented. We take this opportunity to point out several critical flaws in their study. This article is protected by copyright. All rights reserved.