When a fast moving stream meets a slow moving stream or a still body of water, it deposits its load of sediment in the slow moving stream. This is how the tributaries to the Upper Mississippi built its floodplain. Rivers, like the Wisconsin, deposited sediment eroded from its uplands in the slow moving Mississippi, creating its alluvial fan or delta. The Mississippi itself deposited the sediment eroded from its basin in the Gulf of Mexico and built the Louisiana coast over the last 7,000 years.
Downstream from its tributaries, the Mississippi shaped the sediment into C-shaped island, eroding sediment from the head of each island and depositing it at its foot. In this way the islands moved downstream.
You can get a good view of the C-shaped islands in the Mississippi from Pike’s Peak State Park, not far from McGregor, Iowa. The arms of the C protected shallow lakes, spawning places for fish, nesting places for ducks.
The construction of the 9-foot navigation channel and the locks and dams that made it possible stopped the process of erosion and deposition and locked the islands in place. The river, which moved sediment downstream very slowly before the dams, stopped moving sediment delivered to it by its tributaries all together. The interior lakes began to fill with silt.
McGregor is a long interior lake–six acres, built as the C-shaped island that surrounds and defines it eroded downstream. Two channels bind the island, navigation channel on the west and East Channel on the east and south.
Like so many of the V-shaped islands in the Upper Mississippi McGregor Lake is silting in. Add that to steady water levels behind Lock and Dam 10 and the lake is losing its value as habitat for fish and water birds.
Anglers who fish for northern pike, large mouth bass, and panfish in East Channel and McGregor Lake complained that snags in East Channel make it difficult to navigate at low water. The snags and other woody plants in the channel, however, are valuable fish and waterbird habitat.