Anthropogenically-intensified erosion in incising river systems.

Abstract

Anthropogenic alterations to landscapes, such as agriculture, can lead to accelerated erosion in incising (or incised) rivers threatening infrastructure and property, and causing unnaturally high sediment loads, which threaten ecosystems. Steep bluffs and ravines are characteristic landforms in incising river systems, and by understanding erosion on these landforms we can begin to mitigate the impacts of the altered landscape. The Le Sueur River watershed, in southern Minnesota, provides an ideal location for studying the impacts of agricultural land-use on erosion in an incising river. Agriculture in this watershed is made possible through the use of tile drains, which remove water from the uplands and route it directly into ravines or the river, reducing the water that pools on the landscape and increasing flows in the river. Using terrestrial laser scanning (TLS) and aerial photographs, bluff erosion rates in the Le Sueur River were measured and used to develop a watershed scale sediment budget. In the Le Sueur River bluffs account for 56 ± 12% of the 2000-2010 average measured total suspended solids load. These data were also used to interpret how changes in land-use and climate have accelerated bluff retreat in this watershed. The data collected paired with field observations show that over-steepening at the bluff toe drives bluff retreat, yet weakening due to groundwater seeps and freeze-thaw also contributes to erosion. The increases in flow rates and volumes brought on by tile drainage in this watershed have resulted in increased bluff erosion. To study ravine response to changing overland flow hydrology, brought on by tile drainage, small physical experiments were used to measure how changing the delivery rate of a fixed volume of water impacts erosion. The results of these experiments showed that regardless of flow rate the volume of sediment removed remained the same, suggesting that the tile drains installed in the Le Sueur River watershed may have decreased ravine growth. Results of each of these projects independently improves our understanding of bluff erosion and ravine growth processes, yet combined they provide insight into how changing hydrology impacts erosion throughout an incising watershed. While agricultural landscape alterations, especially tile drains, have decreased ravine growth they have resulted in increased bluff erosion. Because bluffs in the Le Sueur River watershed account for more than half of the total sediment load, there is a net increase in sediment loads as a result of anthropogenic landscape alterations.