ABSTRACT
A valley-bottom gully becomes a valley-head gully as its head scarp migrates into the valley head. Valley-side gullies usually result from less concentrated flow coming from diverse directions. This chapter discuses the dynamics and processes of gullying that occur on a short-term basis, with some implications for the long term. It describes gully development and examines the concept of geomorphic thresholds as applied to the processes of valley-bottom gullying in small, agricultural drainage basins in the loessial soils regions of western Iowa and Missouri. Gully erosion rates at the Treynor watersheds were measured, during periods of surface runoff, by sampling the sediment content of stream flow at two locations—immediately upstream and about 100 m downstream from each gully headcut. The dominant mode of gully growth within the thick loessial soils study area is by mass wasting of the walls, with most of the soil loss coming from the vicinity of the retreating headscarp.
