In order to properly manage the Tualatin River and avoid future violations of the State dissolved oxygen standard, it is imperative to understand the relative magnitudes of the various sources and sinks of dissolved oxygen in the river. Possible sources of DO include reaeration (exchange with the atmosphere) and photosynthetic production. Potentially important sinks of DO include water-column biochemical oxygen demand (BOD), sediment oxygen demand (SOD), ammonia nitrification, and the respiration of phytoplankton and zooplankton.
SOD is a combination of all of the oxygen-consuming processes that occur at or just below the sediment/water interface. SOD is partly due to biological processes and partly due to chemical processes. Most of the SOD at the surface of the sediment is due to the biological decomposition of organic material and the bacterially facilitated nitrification of ammonia, while the SOD several centimeters into the sediment is often dominated by the chemical oxidation of species such as iron, manganese, and sulfide (Wang, 1980; Walker and Snodgrass, 1986).
SOD has been found to be an important sink for dissolved oxygen in a wide variety of surface waters. In Oregon, Thomas (1970) measured SOD rates at nine sites in the lower Willamette River and found rates as high as 19.5 g/m 2 d at sites heavily impacted by pollution. Caldwell and Doyle (1995) recently found SOD rates in the same reach of the lower Willamette River to range from 1.3 to 4.1 g/m 2 d, indicating a substantial improvement in water quality over 25 years. These rates are still high enough, however, to be important sinks for dissolved oxygen (Tetra Tech, 1995).
Prior to 1992, the magnitude of the SOD rate in the Tualatin River was unknown. This unknown SOD was thought to be an important part of the oxygen budget in the Tualatin River, and because the lower reaches of the river often produced very large phytoplankton blooms (greater than 100 µg/L of chlorophyll- a ), it had been hypothesized that the SOD rate in that reach of the river would be heavily influenced by the decomposition of settling algal detritus. Therefore, it was thought that reducing the amount of phytoplankton would result in a concomitant reduction in the impact of the SOD on DO concentrations.
The Unified Sewerage Agency (USA) of Washington County is one of the designated management agencies for the Tualatin River. In order to manage the river for dissolved oxygen, it is necessary for USA managers to know the magnitude of the SOD rate, how it varies spatially and temporally, and whether that rate is influenced by decomposing algal detritus. An ongoing USGS/USA cooperative investigation into the sources and sinks of dissolved oxygen in the Tualatin River was started in 1990. This study of SOD was part of that larger investigation.
This report discusses the USGS measurements of SOD in the Tualatin River Basin. Specifically, this investigation of SOD in the Tualatin River Basin was designed to:
These objectives were accomplished by measuring the SOD rate at various locations and at different times during the low-flow summer period. Other measures of water quality necessary to this study, such as chlorophyll-a concentrations, were collected by USA as part of a routine monitoring program. The investigation was restricted to the low-flow period between May 1 and October 31. Throughout this report, all references to algae refer only to phytoplankton.
The Tualatin River Basin is located in northwestern Oregon on the west side of the Portland metropolitan area (fig. 1). Its 712 square mile drainage area supports a growing population of more than 320,000 and a wide variety of forest-related, agricultural, industrial, and residential uses. The basin is bounded on the west by the Coast Range, on the north and east by the Tualatin Mountains, and on the south by the Chehalem Mountains and by Parrett Mountain. The Tualatin River flows generally from west to east and joins the Willamette River near West Linn. River discharge generally reflects the region's climate and is often augmented during the summer with releases from Henry Hagg Lake. Most of the precipitation falls as rain during the November to April period, resulting in high winter flows on the order of thousands of cubic feet per second (ft 3 /s). The May through October period is much drier; streamflow decreases to its lowest levels in July, August, and September, and typically is less than 200 ft 3 /s during that period.
Figure 1. Location of sediment oxygen demand sampling sites in the Tualatin River Basin, Oregon. (RM, river mile.)
The main stem of the Tualatin River is 79.4 miles long and has its origin in the forested Coast Range on the western edge of the basin. In its head-water reach (RM 79.4 to 55.3), the river is narrow and drops in elevation rapidly with an average slope of 74 feet per mile. Downstream of that reach, the river reaches the bottom of the valley and begins to meander through a predominantly agricultural area. This meander reach extends from RM 55.3 to 33.3; the river there is roughly 50 feet wide and has an average slope of 1.3 feet per mile. From the meander reach, the river flows into its reservoir reach (RM 33.3 to 3.4), spreading out to about 150 feet wide and slowing down with an average slope of only 0.08 feet per mile, depending on the discharge rate. At RM 3.4, the river enters a pool and riffle reach and loses 13 feet of elevation per mile before reaching its confluence with the Willamette River. From the lower end of the reservoir reach to its mouth, the river flows through a mixture of agricultural and urban landscapes.
Major tributaries of the Tualatin River include Scoggins, Gales, Dairy, Rock, and Fanno Creeks (fig. 1). Smaller streams that eventually feed into the river include Beaverton, Bronson, Willow, and Cedar Mill Creeks, among many others. The Scoggins and Gales Creek drainages are primarily forested. Dairy Creek flows primarily through agricultural land. The Rock Creek drainage is mixed, with a large urban influence. Fanno, Beaverton, Bronson, Willow, and Cedar Mill Creeks all flow through predominantly urban areas.
Site locations were selected using several criteria. Main-stem sites were selected for (a) adequate distribution throughout both the meander and reservoir reaches, (b) proximity to monitoring stations, and (c) access considerations such as landowner permission and safety. Tributary sites were selected for (a) sufficient water depth to submerge the measurement chamber, (b) a variety of upstream land uses, (c) an adequate areal distribution, and (d) access considerations.
A number of SOD measurement sites (sites 1-7, fig. 1) were selected in the reservoir reach of the main stem for two reasons. First, that reach is the location of the largest phytoplankton populations. Second, the long residence time and low reaeration rate in that reach tend to increase the impact of those processes that consume oxygen in the water column and at the sediment/water interface; therefore, the SOD was expected to be an important part of the dissolved oxygen budget there. Two sites (8 and 9, fig. 1) were chosen in the meander reach as controls against the influence of the phytoplankton; the algal population in the meander reach typically is very small. Finally, a number of sites (10-20, fig. 1) were chosen on many of the tributaries in order to compare the effects of stream order. At most of the sites, ground water was found to be slowly discharging to the stream at the time of the measurement.
The sediments at all sites in the meander and reservoir reaches of the main stem and most sites on the tributaries are composed mainly of silt and clay-sized particles. Only Gales Creek showed evidence of much sand (visual inspection). In several short reaches of the main-stem Tualatin River, the river bottom does have large rocks and cobbles where the river flows over bedrock sills. Most of the river bed, however, has a clayey bottom that typically is covered with several centimeters to several feet of loose silt embedded with detrital material such as leaves, twigs, and plant material. The river is often blocked by logjams in many locations; even where such logs are not visible from the surface, they are common on the river bed. In some places, the clayey bottom is swept clear of silt by the flowing water. In low-velocity depressions, very fine-grained detrital materials tend to accumulate.
This investigation benefited greatly from a number of people and organizations. We gratefully acknowledge the scientific collaboration and financial contributions of the Unified Sewerage Agency of Washington County and the assistance of John Jackson (USA), Janice Miller (USA), Jan Wilson (USA) and Larry Caton (ODEQ). Measurements in the main stem were assisted by USGS scuba divers Dennis Lynch, Jim Poole, Dave Carlson, Dan Zimmerman, Kurt Carpenter, Jim Caldwell, and Ken Skach; additional assistance was provided by Tamara Wood, Richard Norris, Matt Johnston, Bernadine Bonn, Kris Keller, and volunteers Matt Leve and Tirian Mink. Access to sites on the Tualatin River and its tributaries through private property was kindly given by Howard Grabhorn (Lakeside Reclamation), Jim Peterson (Meriwether National Golf Course), Tim Miller (Roamer's Rest), Darrell Vandehey (Reser's Fine Foods), Jeffery Harris, the Tualatin Hills Park and Recreation District, USA, and the Cities of Hillsboro, Tigard, and Durham.
Sediment Oxygen Demand in the Tualatin River Basin, Oregon, 1992-96
U.S. Geological Survey