Featured Report:

The Michigan Department of Environmental Quality (MDEQ) and the city of Otsego, Mich. are considering the removal of the Otsego City Dam. Removal of the structure will return part of the Kalamazoo River to run-of-river flow and improve aquatic habitat in affected river sections. However, because of PCB contamination in the sediment, the U.S. Environmental Protection Agency (USEPA) has designated the Kalamazoo River from the city of Kalamazoo to its outlet into Lake Michigan as a Federal Superfund site. Removal of the dam may result in a mobilization of contaminated sediment with adverse consequences on the downstream environment.

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The U.S. Geological Survey (USGS), in cooperation with MDEQ and USEPA, studied the volume, character, and size distribution of instream sediments in the Otsego City impoundment on the Kalamazoo River in 2001-2002. Water depth and sediment thicknesses were measured along 85 transects. Sediment cores were collected at 76 sites in the impoundment. Sediment-depth profiles and particle-size analysis of core samples formed the principal basis for an interpretation of the depositional history and lithology in the Otsego City impoundment. The following are major findings and conclusions of the study:

• The pre-dam streambed has been well defined upstream from the Otsego City Dam. The pre-dam streambed easily is identifiable and is composed of very coarse outwash and alluvium material: cobbles, coarse gravel, and lesser amounts of medium to coarse sand. All of the sediment-depth measurements for this study terminated in this coarse pre-dam outwash and alluvium.
• Two types of deposits are present: (1) lacustrine deposits that accumulated after the dam was constructed and (2) recent (1980s to present) alluvial deposits that accumulated on the lacustrine deposits after the impoundment levels were lowered.
• Lacustrine deposits consist of interbedded, organic-rich silt and clay, fine to medium sand, and some gravel. Auger-point data and sediment cores within the main channel show that the thickness of the current (2002) lacustrine deposits ranges from 0 to 7.5 ft in the Otsego City impoundment.
• Post-drawdown alluvium deposits are medium to coarse sand and gravel with traces of silt and clay. This coarse-grained surface is related to increases in gradient and velocity, which took place after water levels were lowered at the Otsego City Dam in the 1980s. These alluvium deposits lie directly on remaining instream lacustrine deposits. Thickness of the post-drawdown alluvium ranges from 0 to 3.0 ft in the main channel of the Otsego City impoundment.
• The combined volumes of post-dam lacustrine deposits and post-drawdown alluvium are estimated to be 457,270 yd³. Total sediment thickness ranges from 0 to 11.2 ft, and average thickness is 3.7 ft over an area of 3,291,470 ft². Approximately 56 percent (258,000 yd³) of the total 457,270 yd³ of sediments in the impoundment is within the main-stem channel between the Otsego City Dam and Plainwell Dam; 44 percent is within the side channels. Of the 258,000 yd³ in the main channel, 93 percent of that is located in the lower half of the main channel.