CHANNEL SHRINKING AND FLOOD PLAIN CONSTRUCTION ON THE FREMONT RIVER NEAR HANKSVILLE UTAH

A series of large floods swept down the Fremont River beginning about 1897, and transformed its narrow meandering channel into a broad braided one. Huge volumes of sediment were swept downstream into the Colorado River system from the reach between Caineville and Hanksville. C.B. Hunt's studies in the 1930's documented the resulting braided channel, at least 1000 feet (300 m) wide and eroded 12 to 15 feet (4 to 5 m) below the former flood plain level.

The Fremont River began to constrict its channel and construct a new vegetated flood plain inset within the former flood channel beginning about 1940. This process accelerated in succeeding decades, until at present the channel averages 50 feet in width, and 60 to 70% of the former flood channel is now vegetated flood plain. Sequential aerial and ground-based photographs and monitoring of cross sections since 1966 have documented this process. We estimate that flood plain construction has been storing 5 to 10 acre-feet of sediment per year per mile of river valley, sediment which would otherwise be carried downstream to Lake Powell.

Flood plain reconstruction has been accompanied by profound changes in the geomorphic character and hydraulic geometry of the channel, and in the density and species composition of the riverbank vegetation. These changes are documented by repeat photography. Ongoing evolution of the system involves a complex set of feedback loops, further affecting channel geometry, flood wave propagation, depth to water table, quality of base flow, density and maturity of vegetation and consequent effective hydraulic roughness, disposition of sediment and probably many unidentified parameters.