The peak-flow hydrology in the Molalla River has been characterized by a series of large floods during the 1960s and 1970s, a period of relatively small peak flows from 1975 to 1995, and a relative increase in severity of events in the past 15 years. Although incomplete, the gaging record for the early 20th century showed only modest high flows. The flood chronology since 1960 has affected the geomorphology of the river corridor, principally by increasing the active-channel width. The area affected by channel migration in the late 20th century, however, was reduced by the construction of revetments along the river corridor which acted to contain channel movement.
The study area along the Molalla River was divided into six unique geomorphic reaches. The upper-most reach, designated GR6, is a narrow, bedrock-controlled reach with ample shade and large riffles. The next downstream reach, GR5, is also largely bedrock controlled but has a wider flood plain and active channel-migration zone. The longest geomorphic reach, GR4, has a wide channel-migration zone with many strategically placed revetments that work in concert with bounding bedrock to the northeast to suppress overall channel movement. In contrast, GR3 is a wide, active reach that responds more dramatically to flood and non-flood periods than the other geomorphic reaches. The anthropogenically confined GR2, adjacent the City of Canby, has relatively little historical channel movement and relatively few gravel bars. Finally, the farthest downstream reach, GR1, is an actively meandering reach that most closely resembles its pre-development state.
Detailed analysis of aerial imagery from 1994, 2000, 2005, and 2009 showed that channel-migration activity and active-channel widths were greater in GR3 than in any other geomorphic reach and were related directly to the timing and magnitude of high flows. Similarly, the revegetation of exposed bars is significant in GR3 and elsewhere when large floods do not occur. A qualitative analysis of older aerial imagery dating back to 1936 showed that the recent channel-migration activity in GR3 is no greater than it was historically. Channel-migration activity in GR2, GR4, and GR5 was reduced relative to historical rates as a consequence of the construction of revetments and encroachment along the river corridor.
Analyses of the longitudinal water-surface profile first suggested a possible accumulation of alluvium in GR3, but subsequent analysis of the shape of the longitudinal profile juxtaposed against bedrock outcrops in the river channel showed that the river is largely flowing over a shelf of bedrock and not filling with sediment.
Water-quality, benthic algae, and benthic invertebrate conditions were examined during summer low-flow periods to determine the overall health of the river and to provide possible insights into the physical or chemical influences on diatom assemblages.
A wetter than normal spring in 2010 resulted in higher-than-normal flows in July and August that may have delayed the algal growing season and limited the accrual of algal biomass in the river. Longitudinal changes in water quality, including downstream increases in water temperature and specific conductance, were observed in the Molalla River during August and September. Such patterns are typical of many rivers receiving inputs from anthropogenic sources in the flood plain, including agricultural and rural residential lands (Milk and Gribble Creek basins) as well as some urban runoff in the lower river.
Nutrient concentrations in the Molalla River were generally low at most sampling sites but did increase at the Goods Bridge and Knights Bridge sites, presumably from a greater influence from anthropogenic sources that enter the river from tributaries, agricultural irrigation returns, or groundwater in the lower basin. Nitrate concentrations at Glen Avon and Knights Bridges exceeded their respective reference values for streams in the Cascade Range and Willamette Valley. Although the nitrate-nitrogen concentrations were somewhat elevated, phosphorus, in contrast, is relatively much less abundant in the Molalla River. N:P ratios for soluble, biologically available nitrogen and phosphorus were lower in the upper middle reaches (less than 5), but the absolute concentrations of orthophosphorus (0.010 milligrams per liter or less in July) suggest that attached periphytic algae in the river may be limited by phosphorus concentrations or some other factor, but probably not by nitrogen. The Molalla River has lower phosphorus concentrations than other rivers draining the Cascade Range because the phosphate-rich rocks of the Oregon High Cascades, prevalent in other drainages, are not present in the Molalla River basin, which is wholly contained within the Western Cascade Range geologic province.
The 2010 algal growing season was delayed due to an unusually cold and wet spring, which produced streamflows 12–18 percent higher than normal in July and August and could have limited the accrual of periphyton biomass in the river. Nevertheless, a healthy biofilm of diatoms and other types of algae developed in the shallow riffle habitats during July, covering the entire stream channel in some areas. Generally, riffle habitats appeared healthy, with little sediment and low substrate embeddedness (that is, the degree of infilling of fine sediments around gravels and cobbles) was less than 5 percent at all sites except the Knights Bridge site, where embeddedness was about 10 to 25 percent higher.
Algal biomass levels in July were moderate, ranging from 30 to 55 mg of chlorophyll-a per square meter, and the high densities of benthic macroinvertebrate grazers in the riffles suggests that the accumulation of algae (biomass levels) may have been limited by these herbivores. In August, however, a benthic bloom of filamentous green algae (Cladophora glomerata) increased algal biomass in the lower river, with nuisance levels at the Knights Bridge site. Higher nutrient concentrations (both nitrate and orthophosphate) combined with fewer invertebrate grazers (mostly snails) likely contributed to the higher biomass at this site. Long filaments of Cladophora also were observed in the area near the Canby drinking-water treatment plant, where in previous years, algae have clogged water intakes during periods of senescence when algae detach from the river bed and enter the intake. In 2010, algal biomass conditions were not as severe and the intakes were not affected.
Distinct fluctuations in concentrations of dissolved oxygen and in pH levels from algal photosynthesis were observed at all sites sampled, with the largest diel changes and highest daily maximum values occurring at the two most downstream sites, particularly at Knights Bridge. Although some relatively high pH values were measured (as much as 8.4 units), none of the pH measurements exceeded State of Oregon water-quality standards, even in the afternoon hours on warm sunny days. Dissolved oxygen concentrations at Goods Bridge and Knights Bridge did not meet the 8 milligrams per liter criteria in the early morning hours, but compliance with the standards is only evaluated with 30-day average minimum values, which were not available. Relative to the salmon spawning criteria, for which the data collected during this study applies only to the Glen Avon Bridge site in September, water temperature, pH, and concentrations of dissolved oxygen all met the state standard in effect.
Thirty-three species of algae were identified in the Molalla River, including fast growing small diatoms and very large stalked diatoms, filamentous green and blue-greens, and a few planktonic forms of green and blue-green algae that may have washed into the river from an upstream pond. The occurrence of high-biomass forming types of algae in the river, including filamentous greens such as Cladophora and large stalked diatoms such as Cymbella and Gomphoneis, could be a concern for fish populations because of the potential for smothering fish redds or by impacting benthic invertebrate populations that feed fish.
Together, most of these algae (and overall algal biomass) are typical of generally high quality waters with little organic pollution, high concentrations of dissolved oxygen, and alkaline pH. The relatively high percentage of eutrophic taxa does, however, suggest some degree of nutrient enrichment in the river, despite the relatively low concentrations observed at most sites. Uptake of dissolved nutrients by algae, and inputs of additional nutrients, complicates interpretations regarding nutrient concentrations in the river, especially because samples were collected during summer growing season.
Although the bulk of the diatom species generally were similar among at least the four upstream sampling sites, the multivariate ordination suggests a downstream trend in assemblage structure from the Glen Avon Bridge site to the Highway 213 Bridge. The next downstream site, at Goods Bridge, near the downstream end of the alluvial GR3 reach, however, plotted closer to the most upstream site at Glen Avon Bridge, which indicates a change in assemblage structure. The algal indicator species analysis showed a change in species composition at the Goods Bridge site, including decreases in eutrophic diatoms, increases in the relative abundance of oligotrophic diatoms, and an increase in diatoms sensitive to organic pollution that suggests an improvement in water quality conditions. Although this may be related to the enhanced water exchange into and out of the streambed in the alluvial reach, and such hyporheic activity could work to clean the river of organic compounds and nutrients, small decreases in water quality (lower concentration of dissolved oxygen, and higher conductance and nutrient concentrations) were observed between the Highway 213 and Goods Bridge sites.
The multivariate analysis relating the diatom species composition data to the geomorphic and water-quality variables indicated that the presence of local gravel bars, bedrock, exposure to the sun (open canopy), and pH had a significant role in shaping the diatom assemblage structure. Although there was a high percentage of similarity among samples, many of these factors have the potential to affect diatoms and other algae through various interrelated mechanisms that relate to channel mobility and associated effects on light available for algal photosynthesis, for example, and other potential factors.
Although only qualitatively addressed for this study, benthic macroinvertebrates, including mayflies, caddisflies, and stoneflies, were abundant in the Molalla River and indicate a high degree of secondary production in the riffles throughout the study reach. Snails, another voracious grazer of algae, also were relatively abundant at the Goods Bridge and Knights Bridge sites. Additionally, large numbers of the large caddisfly larvae Dicosmoecus were observed throughout most of the lower river in a range of depths and habitats. The large densities of these grazers, combined with the moderate level of algal biomass, suggest that invertebrate grazers could have limited the accrual of algae during summer 2010, an assertion that could be evaluated with further study. In northern California’s Eel River, high abundances of Dicosmoecus were detected in summers following winters that lacked bankfull flow, as was the case for the Molalla River in water year 2010. The lack of disturbance might explain the high abundance of these herbivores in the Molalla River.
The information from this study can be used to adapt management strategies for the Molalla River and its flood plain. These strategies may assist in developing and maintaining a healthy river environment that includes high-quality water for aquatic life and human consumption.
This report includes chapters that summarize the results of multidisciplinary studies to quantify and characterize the current (2011) status and baseline conditions of the lower Elwha River, its estuary, and the adjacent nearshore ecosystems prior to the historic removal of two long-standing dams that have strongly influenced river, estuary, and nearshore conditions. The studies were conducted as part of the U.S. Geological Survey Multi-disciplinary Coastal Habitats in Puget Sound (MD-CHIPS) project.
Chapter 4 details a series of hydrological data collected by the MD-CHIPS Elwha project. These include groundwater monitoring, surface water-groundwater interactions in the estuary, an estimated surface-water budget to the estuary, and a series of temperature and salinity measurements.
Rivers draining Mount Rainier carry heavy sediment loads sourced primarily from the volcano that cause acute aggradation in deposition reaches as far away as the Puget Lowland. Calculated yields ranged from 2,000 tonnes per square kilometer per year [(tonnes/km2)/yr] on the upper Nisqually River to 350 (tonnes/km2)/yr on the lower Puyallup River, notably larger than sediment yields of 50–200 (tonnes/km2)/yr typical for other Cascade Range rivers. These rivers can be assumed to be in a general state of sediment surplus. As a result, future aggradation rates will be largely influenced by the underlying hydrology carrying sediment downstream. The active-channel width of rivers directly draining Mount Rainier in 2009, used as a proxy for sediment released from Mount Rainier, changed little between 1965 and 1994 reflecting a climatic period that was relatively quiet hydrogeomorphically. From 1994 to 2009, a marked increase in geomorphic disturbance caused the active channels in many river reaches to widen. Comparing active-channel widths of glacier-draining rivers in 2009 to the distance of glacier retreat between 1913 and 1994 showed no correlation, suggesting that geomorphic disturbance in river reaches directly downstream of glaciers is not strongly governed by the degree of glacial retreat. In contrast, there was a correlation between active-channel width and the percentage of superglacier debris mantling the glacier, as measured in 1971. A conceptual model of sediment delivery processes from the mountain indicates that rockfalls, glaciers, debris flows, and main-stem flooding act sequentially to deliver sediment from Mount Rainier to river reaches in the Puget Lowland over decadal time scales. Greater-than-normal runoff was associated with cool phases of the Pacific Decadal Oscillation. Streamflow-gaging station data from four unregulated rivers directly draining Mount Rainier indicated no statistically significant trends of increasing peak flows over the course of the 20th century.
The total sediment load of the upper Nisqually River from 1945 to 2011 was determined to be 1,200,000±180,000 tonnes/yr. The suspended-sediment load in the lower Puyallup River at Puyallup, Washington, was 860,000±300,000 tonnes/yr between 1978 and 1994, but the long-term load for the Puyallup River likely is about 1,000,000±400,000 tonnes/yr. Using a coarse-resolution bedload transport relation, the long-term average bedload was estimated to be about 30,000 tonnes/yr in the lower White River near Auburn, Washington, which was four times greater than bedload in the Puyallup River and an order of magnitude greater than bedload in the Carbon River. Analyses indicate a general increase in the sediment loads in Mount Rainier rivers in the 1990s and 2000s relative to the time period from the 1960s to 1980s. Data are insufficient, however, to determine definitively if post-1990 increases in sediment production and transport from Mount Rainier represent a statistically significant increase relative to sediment-load values typical from Mount Rainier during the entire 20th century.
One-dimensional river-hydraulic and sediment-transport models simulated the entrainment, transport, attrition, and deposition of bed material. Simulations showed that bed-material loads were largest for the Nisqually River and smallest for the Carbon River. The models were used to simulate how increases in sediment supply to rivers transport through the river systems and affect lowland reaches. For each simulation, the input sediment pulse evolved through a combination of translation, dispersion, and attrition as it moved downstream. The characteristic transport times for the median sediment-size pulse to arrive downstream for the Nisqually, Carbon, Puyallup, and White Rivers were approximately 70, 300, 80, and 60 years, respectively.
Analysis of stage-discharge relations at streamflow-gaging stations along rivers draining Mount Rainier demonstrated the dynamic nature of channel morphology on river courses influenced by glaciated, volcanic terrain. The greatest rates of aggradation since the 1980s were in the Nisqually River near National (5.0 inches per year) and the White River near Auburn (1.8 inches per year). Less pronounced aggradation was measured on the Puyallup River and the White River just downstream of Mud Mountain Dam. The largest measured rate of incision was measured in the Cowlitz River at Packwood (5.0 inches per year).
Channel-conveyance capacity estimated using a one-dimensional hydraulic model decreased in some river reaches since 1984. The reach exhibiting the largest decrease (about 20–50 percent) in channel-conveyance capacity was the White River between R Street Bridge and the Lake Tapps return, a reach affected by recent flooding. Conveyance capacity also decreased in sections of the Puyallup River. Conveyance capacity was mostly unchanged along other study reaches. Bedload transport was simulated throughout the entire river network and consistent with other observations and analyses, the hydraulic model showed that the upper Puyallup and White Rivers tended to accumulate sediment. Accuracy of the bedload-transport modeling, however, was limited due to a scarcity of sediment-transport data sets from the Puyallup system, mantling of sand over cobbles in the lower Puyallup and White Rivers, and overall uncertainty in modeling sediment transport in gravel-bedded rivers. Consequently, the output results from the model were treated as more qualitative in value, useful in comparing geomorphic trends within different river reaches, but not accurate in producing precise predictions of mass of sediment moved or deposited.
The hydraulic model and the bedload-transport component were useful for analyzing proposed river-management options, if surveyed cross sections adequately represented the river-management site and proposed management options. The hydraulic model showed that setback levees would provide greater flood protection than gravel-bar scalping after the initial project construction and for some time thereafter, although the model was not accurate enough to quantify the length of time of the flood protection. The greatest hydraulic benefit from setback levees would be a substantial increase in the effective channel-conveyance area. By widening the distance between levees, the new floodplain would accommodate larger increases in discharge with relatively small incremental increases in stage. Model simulation results indicate that the hydraulic benefit from a setback levee also would be long-lived and would effectively compensate for increased deposition within the setback reach from increased channel-conveyance capacity. In contrast, the benefit from gravel-bar scalping would be limited by the volume of material that could be removed and the underlying hydraulics in the river section that would be mostly unaffected by scalping.
Finally, the study formulated an explanation of the flooding that affected Pacific, Washington, in January 2009. Reduction in channel-conveyance capacity of about 25 percent at the White River near Auburn streamflow-gaging station between November 2008 and January 2009 was caused by rapid accumulation of coarse-grained sediment just downstream of the gage, continuing an ongoing trend of aggradation that has been documented repeatedly.