Reducing High Flows and Sediment Loading through Increased Water Storage in an Agricultural Watershed of the Upper Midwest, USA

Nate Mitchell,Brent Dalzell,Se Cho,Karthik Kumarasamy,Patrick Belmont,Karen Gran

doi:10.3390/w10081053

Abstract

Climate change, land clearing, and artificial drainage have increased the Minnesota River Basin’s (MRB) stream flows, enhancing erosion of channel banks and bluffs. Accelerated erosion has increased sediment loads and sedimentation rates downstream. High flows could be reduced through increased water storage (e.g., wetlands or detention basins), but quantifying the effectiveness of such a strategy remains a challenge. We used the Soil and Water Assessment Tool (SWAT) to simulate changes in river discharge from various water retention site (WRS) implementation scenarios in the Le Sueur watershed, a tributary basin to the MRB. We also show how high flow attenuation can address turbidity issues by quantifying the impact on near-channel sediment loading in the watershed’s incised reaches. WRS placement in the watershed, hydraulic conductivity (K), and design depth were varied across 135 simulations. The dominant control on site performance is K, with greater flow reductions allowed by higher seepage rates and less frequent overflowing. Deeper design depths enhance flow reductions from sites with low K values. Differences between WRS placement scenarios are slight, suggesting that site placement is not a first-order control on overall performance in this watershed. Flow reductions exhibit power-law scaling with exceedance probability, enabling us to create generalized relationships between WRS extent and flow reductions that accurately reproduce our SWAT results and allow for more rapid evaluation of future scenarios. Overall, we show that increasing water storage within the Le Sueur watershed can be an effective management option for high flow and sediment load reduction.

Highlights

European-American settlement across the Midwestern United States led to widespread wetland drainage and land use conversion to agriculture [1], with important consequences for the hydrology of agricultural regions [2,3,4,5]
Quantifying the collective downstream effects of water retention sites on both flood magnitudes and sediment loading can involve a great amount of uncertainty, and decision-makers need to know the plausible effectiveness of such water retention sites to better evaluate different sediment reduction strategies [11]
The water retention site (WRS) used here (Figure 1A) are defined as (1) topographic depressions determined by the difference between filled and unfilled 9-m digital elevation models (DEMs) derived from 3-m lidar data with (2) specific land use types defined by the National Land Cover Database (NLCD) 2011 layers, (3) relatively high compound topographic index (CTI) values, (4) areas over 3000 m2, and areas not featuring either (5) sites from the Fish and Wildlife Service’s wetland inventory for the conterminous United States (CONUS) or

Summary

Introduction

European-American settlement across the Midwestern United States led to widespread wetland drainage and land use conversion to agriculture [1], with important consequences for the hydrology of agricultural regions [2,3,4,5]. An alternative approach is to decrease the high flows responsible for bank and bluff erosion by retaining water in the landscape longer in an effort to desynchronize stormflow hydrographs and reduce high flows In some cases, such a distributed hydrologic approach to sediment loading reduction may be more effective, economical, and provide additional habitat benefits in the landscape and stream network. Fine-grained till bluffs in the knickzone contribute large quantities of sediment to the channel network [7,21,22,23,24] This transient adjustment created naturally high sediment-loading rates, but sedimentation records from Lake Pepin, a naturally dammed lake further downstream on the mainstem Mississippi River (Figure 1B), show that sediment loads have increased by an order of magnitude since 1830 CE [25]. Most of this increase originated in the MRB [25,26,27], with the Minnesota River’s contribution to Lake Pepin’s sediment loads increasing from 83.9% (±1.1) to 90.0% (±1.4) [26]

Methods

Results

Discussion

Conclusion