Abstract

Preliminary investigation into the feasibility and benefits of removing 5 low head dams located on the Grand River, in the city of Grand Rapids, Michigan, is currently underway. The anthropomorphic hydraulic controls (dams), constructed in the late 1800’s, were built at the location of several bedrock exposures which served as natural hydraulic controls. Prior to dam construction, an abrupt change in river gradient at these exposures resulted in the rapids for which Grand Rapids is named. Evaluation of several alternatives for restoring more natural flow and sediment dynamics in the Grand Rapids reach is part of the removal effort.

This study provides a detailed explanation of the geomorphic setting and history of the entire Grand River, including new mapping and sediment data for five natural hydraulic controls that were identified during preliminary investigation of the region. These controls were confirmed through bathymetric mapping of a ~13 kilometer reach upstream of Grand Rapids between Ada and Lowell, Michigan. A 135 meter long and roughly 30 meter wide exposure of boulder-rich fluvial sediment was identified 5 kilometers upstream of Ada, Michigan. The exposure trends generally N-S and contains fine sand to large boulders. The surface of the exposure possesses a D₅₀ of 87.6 mm and a D₁₀ and D₉₀ of and 12.2 mm and 1,302 mm, respectively. The subsurface of this deposit has a D₅₀ of 14.8 mm and a D₁₀ and D₉₀ of 7.3 mm and 95.5 mm, respectively. The top of the deposit is not flat. Surveying indicates the elevation of the top of the deposit varies by up to a meter. This exposure provides unique substrate and habitat uncommon in the Grand River, which is primarily a sand and silt dominated river. Further mapping and sampling of this exposure may provide data which will allow this reach to be used as an analogue for restoration efforts in Grand Rapids.

The Grand River Valley (GRV), through which the lower Grand River flows, is significantly larger than the modern floodplain. Previous research has suggested that the GRV was formed, since the last glacial maximum (LGM), by glacial outwash travelling from the Huron Basin to Glacial Lake Chicago (in the Lake Michigan Basin). Mapping and analysis of approximately 40,000 water wells adjacent to the Grand River Valley revealed: 1) a bedrock channel, presumably occupied by the ancestral Grand River; 2) evidence for a Grand River outlet north of the modern location which predates the LGM; and 3) a N-S trending area of thick alluvium and boulder occurrence which may represent valley fill of the pre-LGM bedrock valley.