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Scientific Investigations Report 2012–5017


Geomorphic Setting, Aquatic Habitat, and Water-Quality Conditions of the Molalla River, Oregon, 2009–10


Implications for Resource Management


Multiple river-management options are available to resource managers tasked with addressing geomorphic, aquatic habitat, and water-quality issues in the Molalla River. These options differ in complexity, cost, and impact. More importantly, the efficacy of different strategies is strongly dependent on the geomorphic nature of the reach of interest. For example, one strategy may be best applied to GR3 while a separate strategy is best applied to GR5. Although it is outside the purview of the USGS to recommend specific river-management options, restoration engineers and scientists could use this report to guide design and implementation of future action plans (Runyon and Stout, 2011). 


Recent overbank flooding, channel migration, and channel avulsions in select reaches have caused problems for property owners living along the Molalla River flood plain. The geomorphic assessment described herein showed that high flows, including the 1996 flood, have a strong effect over river channel processes, including channel migration rates, and on riparian vegetation that impact stream conditions. Less shade and channel widening cause warming in the river that promotes the growth of bacteria, fish parasites, and disease, which is exacerbated by the low streamflow and high temperatures in summer. The average annual low daily mean streamflow at Canby is just 1.7 m3/s (60 ft3/s), and daily mean streamflow has been as low as 0.6 m3/s (22 ft3/s in 1959). Because the watershed lacks high-elevation snowpack and there are no large reservoirs for water storage in the basin, summer low flows pose significant risks to fish and other aquatic life. 


Looking into the future, it is unclear whether high flows would increase or decrease in severity. It has been suggested that high flows have increased in intensity as warmer global temperatures have more efficiently converted ocean evaporation into rainfall (Min and others, 2011). Along the west coast of Washington State and British Columbia, Mass and others (2011) showed increasing trends in extreme precipitation and associated peak flows from the 1950s to the present. In the same study, however, peak flow trends in the Coastal Range of Oregon were shown to be decreasing over the same period (Mass and others, 2011). A closer investigation of data from 11 streamflow-gaging stations in the Umpqua River basin in the Oregon Coast Range by Wallick and others (2010b) showed a general decreasing trend in peak flow magnitude over most of the 20th century. Regardless of the peak-flow trends along the Molalla River in the coming decades, it is reasonable to anticipate events rivaling the magnitudes of the 1964 and 1996 floods. 


Changes in land use, both along the river corridor and in the upper catchment, may have a stronger impact on river processes than changes in climate. For example, the building of additional houses in the flood plain and the channel-migration zone could lead to increased confinement of the river corridor, as well as higher risk to the structures from natural river processes. Planners evaluating new structures or revetments in the river corridor should consider potential adverse effects on flooding potential, water quality, or aquatic habitat. Similarly, there is an opportunity with modern forestry practices in the upper catchment to minimize the input of sediment into the river corridor and perhaps reduce the flashiness of high flows in the Molalla River.


This study found moderately elevated levels of algal productivity and diel fluctuations in dissolved oxygen and pH symptomatic of excessive photosynthesis, but pH values were compliant with water-quality standards. Dissolved oxygen concentrations, however, could approach levels that are harmful for fish or developing fish eggs and larvae in years when algal abundance or stream temperatures are higher. Because standards are based on 30-day minimum concentrations, evaluation of the data collected for this study against the DO standard was not possible.


Another potential implication for river and fish management is that annual cycles of algal growth and accrual of biomass that provide a base to the food chain are highly variable from year to year depending on weather conditions, flows during colonization, and many other factors. Several years of monitoring over a range in conditions may be necessary to characterize the dominant processes well enough to inform conceptual and mathematical models for prediction purposes. 


In 2010, the Molalla River had only moderate algal biomass and the potential for substantial grazing by herbivorous macroinvertebrates was clearly evident. However, studies in the Eel River in northern California demonstrated that benthic macroinvertebrates may be reduced by large flow events, leading to subsequent increases in algal biomass (Wooton and others, 1996; Powers and others, 2008). Given the potential importance of these processes in the Molalla River, future monitoring and focused studies could provide insights into river food webs, fish health, and productivity. Monitoring and experimental research could help develop new policies, guide adaptive management strategies, and establish realistic expectations for salmon recovery efforts according to hydrologic conditions. Such an understanding may also increase our ability to predict fish returns or anticipate effects of climate change and changes in streamflow conditions.


Options available for improving water quality in the Molalla River include targeted riparian tree planting to increase shade and reduce erosion of soils, bank naturalization (grading and stabilizing banks with vegetation to reduce erosion), and nutrient management to curtail non-point sources along the river and within the flood plain. Improving water-quality conditions by increasing flow in the river is challenged by the lack of persistent snowpack or large water storage reservoirs in the basin, but water conservation strategies might increase instream flows. Water allocations for instream flows are junior to most other uses, and withdrawals of water from a large number of wells in the basin for agricultural and domestic purposes may intercept flow to the river, potentially exacerbating the negative effects of low flows during summer.


Future management actions in the basin, including implementation of the recent TMDL for temperature, are anticipated to lower water temperatures and may result in other improvements to water quality over time. Restoration efforts organized by the Molalla River Improvement District, watershed councils, and other citizen groups including Molalla RiverWatch and the Molalla River Alliance may also contribute to improving habitat and water-quality conditions for fish and other aquatic life. 


Issues with flooding in GR3 and elsewhere could be addressed by giving the river a wider corridor to flow within. Removal of levees in GR4 and GR5 could, for example, reconnect the river to overbank riparian areas and might help reduce downstream flood peaks by temporarily storing water during large floods.


First posted February 29, 2012

For additional information contact:
Director, Oregon Water Science Center
U.S. Geological Survey
2130 SW 5th Avenue
Portland, Oregon 97201
http://or.water.usgs.gov

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