Abstract

 The Menomonee River is a tributary of the Milwaukee River, (Figure1), has a drainage area of 35,200 ha. and a wide variety of land uses. The main objectives of this work were to understand the sediment dynamics of the watershed, determine the sources and sinks of sediment and assess the system sensitivity to land use changes. Sediment, flow and geomorphic data was gathered and used to define the sediment budget. Sediment sinks were quantified by using a theoretical sedimentation model, which was validated using bathymetric data. Bed erosion was calculated by comparing river profiles from 1975 and 2000. Bank erosion was quantified by locating the areas of bank erosion (e.g. mass failure, exposed roots, etc) and relating them to bed erosion sites. The bed and bank erosion volume was estimated by multiplying the eroded cross-sectional area by the length of the river section. Sediment yield from the land was determined by relating sediment load to flow magnitude for different seasons, at several tributaries with different land use characteristics. A hydraulic, hydrodynamic and sediment transport modeling system was set up to aid on the watershed management.
  The main sediment sink within the watershed is the lower navigable reach of the Menomonee River. This reach is periodically dredged by the USACE. The estimated deposition in this reach is approximately 10,000 tons/year representing 31% of the yearly load delivered by the Menomonee River watershed. The remaining sediment load settles in the Milwaukee River estuary, Milwaukee Harbor and Lake Michigan. The sources of sediment are: bed erosion, bank erosion and sediment yield from land. Bank and bed erosion contribute 5% of the total load of this watershed while the remainder is generated from urban and agricultural runoff. Sediment yield from land was observed to vary with season, basin size and land use. Larger basin yield proportionally less sediment load, as expected; agricultural lands yield the most sediment per unit area; and summer months produced more sediment load than other seasons. The main source of sediment on this watershed is agricultural and urban runoff. Changes in land use affect sediment yield notably. Approximately 70% of the sediment delivered to the system (or eroded from bed and banks) is transported to downstream areas including the Milwaukee River, Milwaukee Harbor and Lake Michigan. The modeling system provides relevant information and tools for the adequate management of the watershed.