7746

• UCREFRP Catalog Number: 7746
• Author: Stanford, J. A. and J. V. Ward.
• Title: Management of Aquatic Resources in Large Catchments
• USFW Year: 1991.
• USFW - Doc Type: Recognizing Interactions Between Ecosystem Connectivity and Environmental Disturbance.
• Copyright Material: NO

created a trophic bottleneck for kokanee and the fishery collapsed in 1987 - 88. Since
1989, only incidental kokanee spawners have been observed in McDonald Creek and
the bald eagles have dispersed elsewhere (Spencer et al. 1991).
Accelerated Erosion
As in all river basins, soil and other mineral substratum are naturally eroded by
fluvial processes within the Flathead catchment. Owing to the porous nature of the
bedrock substrata and extensive tills of glacial origin, very little overland or sheet flow
occurs except during extreme precipitation events or during periods of intensely
accelerated snowmelt. Streams originate primarily as springbrooks fed by waters that
percolate into substrata from precipitation at higher altitudes. Springbrooks coalesce to
form the drainage network of the catchment. Therefore, most of the sediment loads
carried by the streams and rivers are derived from erosion of stream channels and
banks.
The rate of erosion is determined by channel morphology, slope, relative
erosiveness of streambank substrata and the intensity and duration of spates. Most of
the sediment load in the system is derived from Tertiary shales deposited as valley fill
and Quaternary tills and alluvium. These soils contain nitrogen and phosphorus that
are either contained within organic debris or associated with the clay lattice of the
mineral particles. Therefore, as much as 60% of the annual riverine nutrient load of the
Flathead River may be associated with sediment particles that are transported for short
time periods, most years during spring runoff, when the rivers and streams of the
catchment are flooding. Only about 10% of the nutrients associated with particles can
be assimilated by the biota (i.e., only about 10% of the particulate phosphorus is labile
or bioavailable, Ellis and Stanford 1986, 1988) and much of the load is either
deposited on the river floodplain or into the lakes as a short-term pulse event. In spite
of the low nutrient bioavai lability, the fertilization effect of the particulates eroded and
transported by fluvial processes is significant owing to 1) the oligotrophic nature of the
water bodies and 2) the dominance of the hydrograph and nutrient mass balance of
both rivers and lakes in the catchment by spring runoff.
Clearly, erosion is a natural process that both shapes the catchment landscape
and also to some extent fertilizes patches within the landscape. Natural (e.g., lightning-
caused fire, insect epidemics, beaver (Castor canadensis) and other large herbivore
influences) and human (e.g., road building, clearcutting) deforestation increases the
seasonal and annual variation in water yield, particularly during spring snowmelt
(Hauer and Blum 1991), thereby accelerating erosion of streambanks and increasing
sediment loads. Erosion of road surfaces and berms or stream crossings is of
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