HR: 1340h
AN: H53C-1269    [Abstracts]
TI: Steepness and Concavity Controls on the Expression of Reach-Scale Channel Morphology, Debris Flow Deposition, and the Spatial Distribution of Salmonids in the Pacific Northwest
AU: * May, C L
EM: cmay@seismo.berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: University of California, Berkeley, Department of Earth and Planetary Science, Berkeley, CA 94720 United States
AB: Steepness and concavity indexes derived from the power function relationship between drainage area and channel slope provide a first-order control on (1) the expression of reach-scale channel morphology, (2) runout potential of debris flows, and (3) the spatial distribution of anadromous fish in the Pacific Northwest. Channels steeper than about 10% are typically dominated by the effects of periodic debris flow scour and subsequent accumulation of coarse sediment. Downstream of this area, channels with slopes between 3 to 10% represent a transition from debris flow to fluvial process dominance. In this transitional region of the network, debris flow deposits often form fill deposits that are subsequently incised by fluvial re-working that leads to the formation of step-pool sequences. Such reaches have restricted salmonid access, generally being most favorable to steelhead and cutthroat trout. The stronger the concavity of a channel profile, the shorter the length of this transitional reach. In the Oregon Coast Range, steepness and concavity values are high and the spatial extent of transitional channels is greatly restricted (typically only occurring in reaches with draining areas between 0.5 and 1.5 km$^{2}$). The abrupt change in slope from steep debris flow prone channels to low-gradient pool-riffle and bedrock channels promotes debris flow deposition and fan formation at tributary junctions. In these highly concave basins, a relatively large proportion of the fluvial channel network have gradients below 3% and are accessible to salmonids, resulting in a broad spatial distribution. This broad distribution allows for a spreading of risk that may enhance a population's ability to persist during severe disturbance. In contrast, many catchments in the Klamath Mountains of northern California have high steepness values but low concavity. In this region, the portion of the network occupied by transitional reaches is greatly expanded. Step-pool channels dominate the majority of the fluvial channel network, and occur in reaches with drainage areas ranging from 5 to 70 km$^{2}$. With low concavity the change in slope at tributary junctions is less pronounced and debris flows rarely form discrete fans. Instead, these mass flows continue to travel down steep mainstem channels and alter aquatic and riparian habitats for many kilometers. Because of the high steepness and low concavity in the Klamath Mountains, the spatial distribution of salmonids is severely restricted to a small portion of the network, limiting their resilience to disturbance. We propose that steepness and concavity indexes provide a useful context for classifying basins that express different reach morphologies, fish habitat capacity, and responses to episodic disturbance.
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting