Spatially Variable Historical Alluviation and Channel Incision in West-Central Wisconsin

Abstract

This study employs data from valley-bottom surveys, coring investigations, topographic maps, and aerial photographs to identify and explain spatial variations in historical alluviation along tributary streams in the Buffalo River watershed, an agricultural watershed in west-central Wisconsin. Similar to findings from other agricultural watersheds, the spatial distribution of historical alluvium in the Buffalo watershed reflects the controlling influence of watershed size and valley-bottom width. These two factors are not, however, clearly related to the deposition and accumulation of historical sediment at all sites. An additional significant factor that has affected valley-bottom sedimentation is historical channel incision. Incision of nearly every tributary stream in the watershed has created enlarged channels with considerable capacities to contain floods, keeping them from inundating and depositing sediment on valley floors. This helps explain why relatively large amounts of historical alluvium were deposited along some streams (because they are unincised or were incised relatively recently) and why much less sediment was deposited along others despite similar watershed characteristics (because they were incised early in the historical period). It also helps explain why, along some reaches, quantities of historical alluvium do not decrease in the upstream direction as expected. Apparently, incision by upstream-advancing headcuts allowed more time for alluvium to accumulate at upstream sites, countering the effects of decreasing drainage area. Today, tributary streams in the Buffalo River watershed compose a fairly well-integrated network of incised channels. As a result, tributary streams can now convey floods and sediment downstream more efficiently than they could in the past.

Keywords::

• Driftless area
• historical alluvium
• sediment delivery
• channel incision
• enlarged channels