Abstract:
In- stream water temperature is one of the most important environmental
factors associated with the decline in salmonid populations and their habitats in the
Pacific Northwest. Most ecological restoration practices that attempt to reduce instream
temperatures center on replanting or reestablishing riparian vegetation and
increasing flows. However, in a large floodplain river like the Willamette, restoring
hyporheic flow may also play an important role in reducing water temperature.
The objective of this dissertation is to examine restoration practices that focus
on the reduction of water temperatures in the upper mainstem Willamette River,
Oregon, for the benefit of salmonid species. Economic information is developed to
determine the cost-effectiveness of water temperature reduction strategies. The CEQUAL-
W2 water temperature model is used to simulate the effects of upstream combinations of riparian shading and flow increments on downstream water
temperatures. Costs associated with these strategies are estimated. These costs consist
of the opportunity costs of losing agricultural production, and losing recreation due to
flow releases from up-stream reservoirs. The effectiveness of each strategy in reducing
water temperature and the associated costs are used to construct a cost-effectiveness
frontier.
The results indicate that the cumulative effects of shading and flows have little
impact on temperature reduction downstream. Even when maximum available
resources are allocated to investments in shade and flow increases they cannot lower
summer water temperatures sufficiently to meet the state water temperature standard.
However, the restoration practices selected from the cost-effectiveness frontiers
method do provide ranges of achievable temperature reductions at minimum costs.
These results are thus useful management tools when selecting policies to pursue
water temperature goals.
Hyporheic temperature reduction is also examined. Restoration practices
associated with this type of cooling are done through removal/reconnection of the
obstacles that are necessary to allow the process of re-creating dynamic channel
complexity. The observed significant reduction of summer water temperatures
associated with channel complexity indicates that this approach is more flexible in
reducing temperature than approaches that rely on flow increments or/and shade.
Although the costs associated with the hyporheic approach are substantial, the effects
of such a long-term ecological improvement of floodplain habitat may substantially
benefit salmonid populations.
