The Nature Conservancy: Big Woods Stream Restoration

Erosion and Deposition

Complex topographic and depositional features are associated with the Mississippi River Alluvial Plain. All the features are due to erosion or deposition by the Mississippi and its tributaries except for loess, which was deposited by wind.

Erosion, sediment transport, and deposition can occur in relatively stable conditions are are natural processes essential to the dynamic stability of rivers and streams. When these processes become extreme or out of balance, erosion and transport become degradation (left) and deposition becomes aggradation (right).

Human modifications to the landscape strongly influence triggers to instability, accelerating the erosion potential and altering sediment transport and flow regimes of channels. See below: The Walla Walla River in December of 1964 defies an attempt at channelization and re-creates meanders, point bars, pools, and riffles.

Returning Equilibrium

Benson Slash Creek was once a wooded, meandering stream located in the heart of the great Mississippi River floodplain. Although there are no known historic photos or accounts of this particular drainage, we can speculate on what it used to look like. Our best guess is that Benson was narrow, relatively deep and gently meandered back and forth across its floodplain before it was channelized.

Before human intervention, it was probably what is referred to as a type "E channel". These biologically-rich streams are highly sensitive to changes in sediment and stream flow, and require thick riparian vegetation for channel stability. Benson Creek was systematically straightened, widened, and deepened over sixty years ago causing it to be in a state of disequilibrium (see channel evolution representation above).

By modeling our restoration design off of undisturbed streams in the region, we hope to return this once diverse ecosystem back to a functioning represenative of its original state. You can see below how the designed stream riffle and pool cross-sections closely resemble those from one of our reference sites. Note that the image on the bottom left depicts the channel in its current, altered condition.

   

Please visit EPA's WARSSS website for more information regarding technical tools for approaching impaired watersheds.