Orwell Lake, in west-central Minnesota, is a flood-control, water-management reservoir first impounded in 1953. Subsequent concern about erosion of the shoreline and a lack of knowledge of slope-erosion processes in this part of the upper Midwest prompted this study to identify and quantify the processes there. The processes were measured at selected sites between June 1980 and June 1983.

Erosion of the banks is predominantly by thaw failure and secondarily by rainsplash and sheetwash. In winter, the upland surface adjacent to the lake freezes to a depth of between 1 and 2 m, depending on surface temperature, snow cover, and distance from exposed banks. In late winter, soil aggregates, released by the sublimation of interstitial ice within the banks, begin to accumulate at the base of the slopes, often veneering snowbanks. As soon as thaw begins, slab failure of bank sediment is followed by mudflows and earthflows. Thaw failure at Orwell Lake in the spring of 1982 accounted for 89% of the erosion (824 Mg); the following spring, 78% (746 Mg). Such slope failure is most intense along north-facing banks and considerably less intense on south-facing banks, where more effective desiccation and sublimation reduce soil-moisture content.

Summer rainsplash and sheetwash were responsible for 11%–22% of the total erosion, amounting to 102 Mg in 1981 and 208 Mg in 1982. Although rainsplash is the most consistent process during summer, the occasional storm during which rainwash occurs causes greater total erosion. Erosion by rain increased in each of the three summers, largely because of increased precipitation.

Infrequent massive deep-slip failures (slumps) have occurred at the east end of the lake where a buried clay-rich unit is stratigraphically and topographically positioned to favor such failures. Drought years followed by heavy spring rains probably will result in additional deep-slip slope failures.

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