In 1998, approximately 100,000 CY of sediment was discharged to a 5-mile section of Fawn River, in Steuben County, Indiana as the result of a rapid draw down of an upstream reservoir (the “Event”). Prior to 1998, Fawn River below the Orland dam (“lower Fawn”) was recognized as one of the best-preserved sections of river in the State of Indiana. The Fawn had a deep swift thalweg, high water quality, and a clean gravel hyporheos interconnected with the underlying aquifer.
Unlike most Indiana Rivers, this section of the Fawn had never been ditched and a wide riparian corridor of undisturbed woodlands and wetlands protected it. When the massive sediment flow entered the river in 1998, there was a large fish kill, and mussel and macro-invertebrate colonies were rapidly buried under the silt and sand. The hyper-concentrated flow (> 60% solids) buried the gravel bottom, filled the deep pools and cuts, and reduced connectivity to the underlying aquifer. The filling of the main thalweg also initiated disastrous bank erosion, and over the next several years the Fawn’s banks and trees toppled into the stream. The result was a wide, shallow, slowed, warmed, and mud-choked river with anoxic conditions at the benthic interface, algal blooms, and advancing colonization by invasive aquatic vegetation. Available habitat for fish and macro-invertebrate populations was reduced and the survival of the most sediment sensitive species was uncertain.