Catastrophic Release of Sediment from Orland Reservoir into the Fawn River, Northwestern Indiana, USA: Assessment of Flow, Sediment Dynamics, and Deposits

In 1998 personnel at the Indiana Department of Natural Resources (IDNR) opened the dam on the Orland Reservoir releasing approximately 76,000 cubic meters of sediment into a 4 km reach of the gravel-bed Fawn River. The release caused severe environmental impact. The IDNR admitted opening the gates, but denied that any sediment was released. They claim that mud deposits downstream of the reservoir are natural, similar to mud deposits upstream of the reservoir. The matter has been in litigation for 10 years. The purpose of this study was to determine the origin of the mud deposits and associated depositional processes.

Mud deposits upstream of the reservoir occur as bars and thin layers in slow-moving parts of the channel. Grain sizes are clay to very fine sand, similar to those in the reservoir. Abundant, horizontally oriented plant fragments and clastic sediment form crude layers. Roots are abundant. Features indicate that the clastic sediment was deposited in slow moving or stagnant water, with vegetative layers reflecting yearly accumulations of plant detritus.

The river gravel is supplied by erosion and winnowing of fines from glacial and periglacial bank sediment. Flow modeling indicates that grain sizes up to coarse sand could be carried in suspension during bankfull flow. The distribution of mud deposits downstream of the reservoir as bars, fills of anabranches, and thin layers on the surfaces of gravel bars is inconsistent with typical fluvial processes of mud deposition. The mud deposits range from clay to very coarse sand, with a high proportion of coarse grains. Deposits are homogeneous and lack primary sedimentary structures. Surface vegetation is absent. Evidence indicates that the mud resulted from the catastrophic release of sediment and water from the reservoir, producing a hyperconcentrated flow that resuspended and mixed with coarser river sediment before rapid deceleration and deposition.