Disrupted Sediment Processes
The legacy of land management and river modifications have altered the pattern of sediment distribution in the Deerfield River watershed. Sediment loads in a river can be influenced by many factors including soil type, vegetation cover, land use, climate, topography, and erosion rates. When the natural inputs and distribution processes are interrupted, the river will make channel adjustments through erosive processes like incision, widening and aggregation to seek balance (River Dynamics). Photo by Field Geology Services
“Sediment is the largest impairment within the assessed streams of the Deerfield River watershed.” (A Watershed-Based Plan)
Above: Former dams and mill disrupt, like this one on the South River sediment processes and build up sediment behind them. These sediments are prone to erosion upstream from incision and downstream through a lack of deposition.
Below: At least 67 percent of the South River has been artificially straightened, at one point hosting 30 active mills (Field, 12). Channel straightening can exacerbate flood conditions by increasing flow velocity, erosion down stream, and leave the channel lacking important habitat features like pools and riffles (Field 6). 1871 map annotated by M. Turre and Field Geology Services
Photo by Claire Baglien
Left: A straightened channel on Pumpkin Hollow Brook in Conway is reinforced by a cement wall accelerating flow velocity and potential for erosion downstream.
Channel Straightening Decreases Stream Complexity Channel straightening has been a common practice in New England. Rivers have been artificially straightened in order to control the flow of water, especially around dams, and the Deerfield is no exception. Straightened river channels alter the water flow patterns and disrupt sediment distribution patterns. The artificial straightening of stream banks tends to result in a lack of in-stream boulders, woody debris and overall complexity that would typically slow the flow of water. These naturally occurring in-stream features create pools and riffles that provide important habitat for aquatic life. They also reduce the erosion of banks by breaking up fast erosive water flows before they hit the soil on the bank. Due to the lack of in-stream complexity, straightened stretches of rivers tend to create a quickly moving chute of water with little in the way to slow the velocity of the flow. These high flows aren’t able to spread across the floodplain, which can lead to incised, deeply scoured channels that carry sediment until it can settle out at the next meander. Sediment settles out of water only when that water has a chance to slow.
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