THE EFFICACY OF STREAM POWER AND FLOW DURATION ON GEOMORPHIC RESPONSES TO CATASTROPHIC FLOODING

Considerable research over the past several decades has focused on the role of large floods in producing catastrophic geomorphic impacts. The link has been surprisingly vexing: not all extreme floods generate unusual geomorphic impacts in part due to inherent thresholds of instability, the timing and sequencing of events, and localized controls. Taking advantage of Tropical Storm Irene, which spawned some of the largest recorded discharges in VT, and associated profound geomorphic responses, we present the geomorphic imprint of Tropical Storm Irene flooding within a larger context of fluvial theory concerning the role of, and trade-off between, the magnitude of energy expenditure during a flood and its duration. We address three major research questions: (1) What are the geomorphic effects of a short duration flood in a humid, well-vegetated landscape? (2) What is the relationship between (a) peak stream power, (b) total stream power, and (c) flow duration of stream power above a critical threshold and the types of associated geomorphic responses? And (3) how does the spatial variation of geomorphic effects relate to reach-scale geologic and geomorphic controls? These questions are addressed along the Saxtons River, a 186 km² watershed in southeastern VT, which had an unprecedented peak discharge and rapid hydrograph rise rate, going from < 2.8 m³/s to > 410 m³/s in ~ 12 hours. Because the peak unit stream power of ~ 500 W/m² exceeded the necessary threshold of 300 W/m² (Magilligan, 1992) for at least 20 hrs (total energy expenditure of ~ 12, 367 x 10³ joules), sedimentological effects were profound, including the entrainment (and imbrication) of boulders > 1000 mm and deposition of gravel bars (mean b-axis > ~ 300 mm) across floodplain surfaces > 3 m above the channel bed. However, despite these extraordinary hydrologic drivers, the effects on channel widening were generally limited and localized. These results suggest that even short duration events can have significant –but selected – geomorphic impacts: the limited duration of the flood may not have been of sufficient length to saturate and widen channels, but the peak discharge and associated stream power was sufficient enough to mobilize and deposit extremely coarse caliber bed material.