Evaluation of Flood-Caused Geomorphic Changes at Restored Stream Channels, Colorado USA

In recent decades, interest in the aesthetics, function, and health of riverine systems has led to the development of an industry devoted to river reconfiguration, rehabilitation, and restoration. Reasons cited for channel reconfiguration include restoration to more "natural" or historical conditions, improvement of water conveyance in flood-prone areas, mitigation of unstable streambed and streambanks, increasing sediment transport, and enhancement of riparian habitat or recreation. Many kilometers of stream and river channels in Colorado have been reconfigured by private entities and resource-management agencies by using designs based on different geomorphic philosophies and classification schemes. However, geomorphic response to, and the effectiveness of, these modifications over a period of time have not been assessed in a consistent manner.

Monitoring and performance assessment are essential, though often overlooked or underfunded, components of river restoration projects. The geomorphic responses of monitored, reconfigured reaches following flood discharges of known magnitude and recurrence interval provide benchmarks against which individual project performance can be assessed. Recent floods in excess of the bankfull discharge have occurred at several monitored reconfigured reaches in Colorado. Geomorphic changes resulting from these floods were evaluated with respect to sediment-entrainment potential and the shear stress generated by the 2-, 5-, 10-year, and observed floods by using a one-dimensional flow model, and by using channel-pattern thresholds based on slope, discharge magnitude, and sediment size. The observed changes included small-scale streambed and streambank erosion and large-scale reach instability, indicating potential design problems for some reconfigured channels when conveying relatively common flood discharges.