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

Between about 1890 and 1910, a series of large floods swept down the Fremont River and transformed its narrow meandering channel into a broad braided one. Huge volumes of sediment were swept downstream into the Colorado River system. C.B. Hunt's studies in the 1930's documented the resulting braided channel, at least 1000 feet (300 m) feet wide and eroded 12 to 15 feet (4 to 5 m) below the former flood plain.

Beginning about 1940 the Fremont River began to narrow its channel and construct a new vegetated flood plain inset within the former flood channel. This process accelerated in the 1970's and 1980's, until at present the channel averages 50 feet in width, and the former flood channel is now 60 to 70% a 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 and channel aggradation is presently storing 5 to 10 acre-feet/yr of sediment per mile of river valley, sediment which would otherwise be carried downstream to Lake Powell. Some of this sediment is derived locally from erosion of the Mancos Shale. Engineering studies by the Utah Division of Water Resources in the 1980's showed that alluvial sediment derived primarily from the Mancos Shale can contain up to 10% soluble salts (mostly gypsum). Its selenium content is unknown.

Geologic studies elsewhere on the Colorado Plateau show that the cycle of erosion (arroyo cutting) and re-filling takes place on the scale of centuries. Thus, under the right combination of natural and cultural conditions, large volumes of sediment and associated salts can be mobilized and swept into the Colorado River system in just a few years. Additional geochemical studies could elucidate the fate of selenium mobilized by weathering of the Mancos Shale and how much is in transient storage in the Fremont River flood plain.