GROUNDWATER EFFECTS ON EVOLUTION OF ENTRENCHED CHANNELS CAUSED BY CHANNELIZATION

An often overlooked factor in the evolution of entrenched channels (caused by channelization) is the role of groundwater. The groundwater flow system is significantly modified by channelization. While aquifer parameters such as recharge rates, area of recharge, thickness, and permeability of the aquifer are not changed with channelization there is a change in base level for discharge of groundwater and changes in groundwater flow path lengths and gradients.

If one assumes that the groundwater flow system was in dynamic equilibrium prior to channelization it is possible to use a modified form of the Dupuit-Forchheimer theory to explore the evolution of the entrenched channel. The concept of effective drainage length is used. If the aquifer is not effectively drained after channelization groundwater sapping processes will act on the channel banks to increased their susceptibility to weathering and erosion. Additionally the lowering of base level will increase the effectiveness of groundwater sapping processes creating a feedback cycle leading to additional entrenchment and/or channel widening and tributary development.

The original meandering channel, because it represents a greater drainage density, more effectively drained the alluvial aquifer system than a subsequent straight channel resulting from channelization. The result is that groundwater sapping processes will drive the development of meanders and tributaries in the straight channel bringing the groundwater system back into a dynamic equilibrium by creating a drainage density that effectively drains the groundwater system. Field sites in southeastern Nebraska will be used as examples.