Potential Future Studies


Future monitoring and focused studies could contribute to better understanding of river health in many ways. Implementation of a monitoring program to evaluate the effectiveness of possible changes in land or water management (flood-plain restoration projects, forestry activities, or water-conservation strategies, for example) might help evaluate and fine tune adaptive-management strategies to improve water-quality conditions.

Future studies also could more fully evaluate the potential for algae to cause high pH or low DO in the river, should conditions deteriorate over time. If this were to occur, nutrient source identification and reduction strategies could be initiated to curb excessive algal production. Basinwide synoptic sampling (see Carpenter, 2003) for nutrients and streamflow in the main-stem Molalla River and tributaries could identify the primary source areas and this information could be useful in developing best management strategies to reduce inputs. 


Although field parameter data collected for the current study helped to characterize water-quality conditions, including daily minimum and maximum values for water temperature, pH, and dissolved oxygen, deployment of a continuous water-quality monitor in the lower Molalla River could provide indications of deteriorating water-quality. Such monitoring could better define the health of the Molalla River through time and analyses of the resulting data could alert managers to potential trends in water quality over time. Continuous data on field parameters also would complement ODEQ’s ongoing ambient water-quality monitoring program, which samples the lower Molalla River several times a year.


Measurements of water temperature, pH, and dissolved oxygen along shorelines in areas of abundant algal growth could characterize the potential negative effects on fish eggs, fry, or juveniles due to adverse water-quality conditions. In addition, sampling of inter-gravel dissolved oxygen concentrations could determine whether ample dissolved oxygen is present in spawning riffles when eggs are incubating. Given that dissolved oxygen levels in redds are typically about 3 mg/L lower than DO levels in the water column, fish egg development could be adversely affected at the lowest DO concentrations observed in the Molalla River (slightly less than 8 mg/L).


Future studies also could examine and quantify algal photosynthesis rates, benthic respiration, and look for key groundwater/surface-water interaction zones in the river. Recent TIR data collected for ODEQ’s temperature TMDL could be used to identify areas where groundwater and (or) hyporheic water emerge. Such studies could provide insights to the importance of alluvial habitats in providing cold water refugia, for example, but could also determine influxes of nutrients that fuel algal growths. The depth of alluvial fill in GR3 could be determined with geophysical approaches or by examining drilling logs for wells in the flood plain. The alluvial section of river, GR3, currently supports abundant benthic macroinvertebrates and appears highly productive, so periodic monitoring of benthic community and riffle habitats for sedimentation and embeddedness, for example, could provide warning signs should conditions change.


Another potentially strong influence on algae populations are benthic invertebrate grazers, which on the basis of high densities, appeared to keep algal biomass levels in check. Although this effect was not investigated or quantified during this study, additional studies could examine the effect of invertebrate grazing on algal biomass and species composition in the Molalla River through enclosure/exclosure mesocosm studies that vary the abundance and (or) composition of grazers, predatory macroinvertebrates, and fish (salmonids and introduced warm water fish), similar to experiments conducted in the Eel River by Powers and others (2008). These multiple year studies highlight the importance of long-term datasets and underscore the need to shift from a single species to whole-community ecosystem perspective when considering management options or developing action plans geared toward fish recovery.


Although nutrient levels in the Molalla River were not very high, uptake by periphyton can mask inputs of nutrient-rich groundwater that may feed algae from below. There is some evidence of water gains in the lower river that may be due to groundwater, and the cooler temperatures in the lower reaches of Gribble Creek also suggest groundwater inputs to the river in this area. Given the agricultural land use in the southwestern part of the Molalla River basin, such inputs could contain elevated concentrations of the nutrients nitrate and orthophosphate, or even agricultural pesticides. Nitrate concentrations were much higher at Knights Bridge than at the Goods Bridge site during both August and September samplings (fig. 28), which could be due to inputs (at Knights Bridge) from groundwater and other sources. Fine-scale seepage studies could be conducted to confirm and quantify these water inputs, and determine nutrient loading from these sources. Given the low nutrient concentrations observed, this might be a logical starting point for identifying nutrient sources if high algal biomass becomes an issue in the future, if drinking water quality deteriorates, or if pH and dissolved oxygen concentrations become unhealthy for fish and other aquatic life.


A full accounting of revetments through the river corridor is an important data set not yet assembled. These data could be used in the planning of restoration projects. A more detailed mapping of the geologic structure of the lower Molalla River, including the identification of geologic units, would aid the understanding of river profile evolution and groundwater/surface-water interactions. Hydraulic and sediment-transport modeling of the river corridor under current and future climate conditions would help planners anticipate shifts to the hydrologic and sediment-transport regime and resulting impacts on fish and people. New bioenergetics models couple river hydraulics with biological factors to evaluate spatially the quality of fish habitat. Applying these models to the Molalla River could provide important insight for fish biologists trying to understand why salmonids do not use the river system as much as other river systems in western Oregon. Finally, thermal models of the river system under future climate-change scenarios would help identify thermal stressors to the aquatic habitat that may be anticipated in the coming decades. 


First posted February 29, 2012