Rates and pathways of recovery for sediment supply and woody debris recruitment in northwestern Washington streams, and implications for salmonid habitat restoration
This dissertation describes a strategy for restoring watershed processes. The restoration strategy is driven by the recognition that (1) salmonids are adapted to local environmental conditions, and (2) spatial and temporal variations in landscape processes create a dynamic mosaic of habitats in a river network. Therefore, the strategy's goal is to restore and maintain the habitat-forming processes to which salmonids are adapted. Comparisons of current rates to historic rates highlight areas where restoration is necessary. The strategy then allows for prioritizing restoration actions. Prioritization does not alter the types of actions that may be taken, but alters the sequence in which they are implemented.Analyses of sediment supply and large woody debris (LWD) recruitment in the context of the natural disturbance regime allow prediction of recovery rates for salmonid habitats. Sediment supply is most significantly affected by changes in mass wasting rates. Stands less than 20 years old have mass wasting rates approximately four times that of mature forest areas, and roads have mass wasting rates approximately 40 times that of mature forest areas. The cumulative effect in a watershed is to increase average annual sediment supply under a forest management regime to about twice that of the natural fire regime. The rate of recovery for sediment supply can be described in terms of average annual travel distance of sediment, which averages about 20 times channel width where channel slope is less than 0.03.LWD recruitment in the study area has been altered by previous logging practices and conversion to agricultural and urban land uses. Compared to projected LWD recruitment under the natural fire regime, forest management typically reduces recruitment of LWD large enough to form pools by an estimated 35 to 100%. Recovery rates for LWD recruitment are primarily a function of channel size and the tree species colonizing a disturbed riparian area. Modeling of LWD recruitment through time predicts that active management of riparian forests will most effectively increase recovery rates in large streams, and that active management is unlikely to improve LWD recruitment in any channel less than 15 or 20 m wide.
- Forestry