Geomorphic impacts of flash flooding in a forested headwater basin

Geomorphic impacts of a July 2001 flood in the Hungry Mother basin near Marion, Virginia, were examined to shed light on the relationships between channel characteristics and the frequency of channel-modifying discharges. Creeks in the study area have been observed for many years, with no significant channel changes since at least 1985. The 2001 flood had a recurrence interval of >200 years, and caused the only channel change, bank erosion, and transport of coarse channel material observed in recent decades. The paucity of fines in channels before or after the flood, and the absence of sub-sand sized material in the flood deposits, indicates that normal, frequent, well-below-bankfull flows are sufficient to transport the (apparently limited) supply of fines. The large particles transported during the 2001 flood after years of inactivity indicate that relatively rare floods are necessary to mobilize this material. This suggests the notion of a bimodal ‘dominant’ discharge. On the one hand frequent flows considerably below bankfull levels are sufficient to maintain the channel and prevent significant accumulation of fine sediments and organic matter. On the other hand, rare floods are necessary to transport the coarser bed material and erode channel banks. In the Hungry Mother area, bed material has no relationship to normal flows, or to flows with recurrence intervals on the order of 1–3 years. Bankfull discharge is apparently not related to either the maintenance or channel-changing dominant discharges. These results suggest that the use of channel dimensions and/or bed material as surrogate indicators of hydrologic regimes can be quite complicated, and that in some streams bankfull flow has no particular significance in terms of sediment transport and channel modifications.