RAVINE CONTRIBUTIONS TO BASIN-WIDE SEDIMENT LOADS IN AN INCISING AGRICULTURAL BASIN, SOUTH-CENTRAL MINNESOTA, USA

Large-scale gullies, termed ravines, are a dominant feature in south-central Minnesota where incising rivers are responding to base level fall associated with excavation of the Minnesota River valley at the end of the last glaciation (13.4 ka). Headward-migrating ravines are the primary connection between incised channels and relatively flat, now-agricultural uplands. This research seeks to understand the role of ravine erosion on fine sediment loading in the turbidity-impaired Le Sueur River basin. We used airborne lidar to map ravine locations, amassing a database of ravine metrics, and giving insight into ravine development over the past 13.4 ka. Historical air photos from 1938 and 2005 were used to map changes in ravine tip location over decadal scales. Finally, we monitored four ravines with auto-samplers for up to three years to compute sediment loads and determine ravine contributions to modern sediment loading.

Minnesota River tributaries are incising via upstream-propagating knick points. Ravines are initiated progressively through time, with the oldest, deepest, and longest ravines located near tributary mouths. However, comparisons of historic ravine tip locations show no systematic tip growth, due to both air photo resolution limits and landowner efforts to slow ravine tip growth. Modern sediment loads were calculated from auto-sampler data on four ravines from 2008-2010. Paired samplers on one ravine indicate most sediment exiting a ravine is sourced within the ravine itself and not the uplands above it. Loads were strongly correlated with incised ravine area. This relationship was used to extrapolate measured loads basin-wide to compare with loads at the river mouth. From 2008-2010, ravines accounted for 2 to 15% of the total sediment load each year. A second analysis quantified the load in the main stem river during times when ravines were active. If all sediment carried by the river came from ravines during those times (a maximum constraint), ravines would still only account for 21% of the total sediment load in 2008 and 3% in 2009, a dry year. Overall, ravines do not appear to account for more than 10-20% of the total sediment budget in the Le Sueur basin. This conclusion stands in contrast to qualitative observations of exceptionally steep ravines with high sediment concentrations in storm events.