Buried in bluff country: Stream and valley sedimentation in the Whitewater River Valley, Minnesota (USA)

Abstract

Erosion and sedimentation are natural and beneficial surface processes but when accelerated by anthropogenic activities related to agriculture yield negative economic and ecologic consequences. European style agriculture spread through 16th to 19th century colonialism has had a profound effect on erosion and sedimentation rates across the globe. The Upper Mississippi River Valley (UMRV) region of the United States, where the Whitewater River Watershed is located, has been particularly affected by European style agriculture implemented in the mid-20th century. From the 1890s through 1920s, increased erosion, sedimentation, and flood frequency precluded land from agricultural production and damaged property and infrastructure in the Whitewater basin. Living and working conditions became untenable and most of the lower valley was abandoned by the 1960s. This geomorphic upheaval invited scrutiny by Soil Conservation Service geologist Stafford Coleman Happ who established a basin-wide sedimentation survey in 1939. He and assistant surveyors established 94 valley transects upon which the distribution and thickness of Euro-American Legacy Sediment (ELS) was measured through auger boring and surface elevation surveys. Repeat field surveys in the 1960s, 1970s, and 1990s were conducted along with an aerial lidar survey in 2008 yielding a robust, over 150-year, record of river valley changes and sedimentation throughout the basin. We use this historical transect sedimentation data to ask how sedimentation rates in the Whitewater watershed have changed since the implementation of soil conservation measures around 1940. Mean sedimentation rates are calculated per transect between floodplain cross-sections or from soil bore depth measurements and are then compared statistically with Welch’s ANOVA and the Games-Howell tests. Between 1939 and 1994, mean transect sedimentation rates for the basin decrease from 0.92 to 0.22 cm/yr. We speculate that this is due to improved soil conservation measures implemented in the basin around 1940. An analysis of land use and land cover change over this time should be completed to corroborate this assumption. The 1994 to 2008 time interval indicates that sedimentation has increased over the last 14 years of this record, but we lack the spatial data coverage to make conclusions about basin wide changes. Still, this change may likely be attributed to the effects of large floods to mobilize sediment, such as the record flood of 2007, as well as anecdotally reported land use intensification since the 1990s. Downstream spatial patterns of sedimentation from the uplands, through the middle gorges of the Whitewater River, to the bluff bordered lower valley appear to follow trends reported in other basins of the UMRV in relation to valley and channel geometry characteristics. These should be investigated in turn to understand how sediment is being routed throughout the river network.