Introduction

Dissolved oxygen is essential to the aquatic health of many rivers and lakes, and the concentration of dissolved oxygen often is used as a water-quality standard to protect fish and aquatic life. The dissolved oxygen (DO) concentration in a waterbody is affected by many processes, such as exchange with the atmosphere (reaeration), photosynthesis by algae or aquatic plants, ammonia nitrification, respiration by algae and bacteria, and the bacterially mediated decomposition of organic matter suspended in the water column or in surficial sediments. The oxygen consumed through the decomposition of organic matter in surficial sediments is called sediment oxygen demand (SOD) and can be one of the most important loss processes for DO, particularly in shallow streams.

Low DO conditions periodically occur in the lower reaches of the Tualatin River and its tributaries in northwestern Oregon (fig. 1), especially during low-flow periods when the water is warm but algal photosynthesis is minimal. During such periods, SOD can account for a large fraction of the total DO consumed. The U.S. Geological Survey (USGS) investigated the effects of SOD in the Tualatin River, measured a median SOD rate of 2.3 grams of oxygen per square meter of sediment per day (g/m²/d) (Rounds and Doyle, 1997), and determined that SOD and water-column oxygen demand are the largest overall sinks for DO in the Tualatin River (Rounds and others, 1999).

In response to water-quality problems in the Tualatin River, Oregon’s Department of Environmental Quality in 1988 adopted a set of Total Maximum Daily Load (TMDL) regulations in an effort to restore the aesthetic qualities of the river and protect the river against low DO concentrations and high pH levels. The TMDLs required substantial ammonia and phosphorus reductions, which were designed to increase DO concentrations and limit the size of algal blooms during the summer months. At that time, it was assumed that algal-derived biomass settling to the river bottom was an important source of decomposable organic matter, and that limiting the size of algal blooms would not only eliminate high pH levels but also decrease the SOD and increase DO concentrations. That assumption was challenged by later measurements that determined that SOD rates generally were not elevated in those reaches of the river that had the largest algae populations, except at one site in the lower river where a slightly higher rate might be ascribed to enhanced deposition of algal biomass (Rounds and Doyle, 1997). Furthermore, water-quality modeling by USGS showed that even if ammonia sources were fully controlled, the Tualatin River still could be subject to periodic low-DO conditions because of DO consumption by SOD (Rounds and others, 1999). Revision of the Tualatin River TMDLs in 2001 recognized the importance of SOD and called for significant decreases (20 percent or more) in the SOD rate of the river and its tributaries through the control of settleable organic materials (Oregon Department of Environmental Quality, 2001).

In order to manage and potentially reduce the effects of SOD, it is important to determine the sources of the organic matter delivered to stream sediments. Potential sources of such organic matter include not only algae, but aquatic plants, soils, terrestrial plant material, and possibly the particulate in effluent from wastewater treatment facilities (WWTFs). Many of these materials, however, may be distinguished from one another through measurements of the content and characteristics of the carbon and nitrogen present in that organic matter. Measurements of stable isotopes of carbon and nitrogen have been used for many years to investigate the nature and sources of organic matter in freshwater systems (Middelburg and Nieuwenhuize, 1998; Finlay and Kendall, 2007).

Stable Isotopes

An element is defined by the number of protons that it contains; for example, all carbon atoms contain six protons and all nitrogen atoms contain seven protons. Atoms of the same element, however, may contain different numbers of neutrons, which cause them to have different masses. Forms of the same element that have different masses are called isotopes; stable isotopes are forms that are not radioactive. Although isotopes vary in the number of neutrons that they contain, their chemical properties essentially are identical. Many elements of biological interest [carbon (C), nitrogen (N), oxygen (O), sulfur (S)] have different isotopic forms. The relative abundances of isotopes can be measured with great precision using a mass spectrometer, and such abundances have been used to study biogeochemical cycles and ecosystem dynamics.

The use of stable isotope abundances is based on the fact that different isotopes of an element participate in the same chemical reactions, but at slightly different rates due to their different masses. Lighter isotopes typically react at a slightly faster rate, which causes the relative isotopic abundance in the reactants and products to differ. This process is called isotopic fractionation. For example, the growth of algae tends to create algal biomass that contains less of the heavier carbon-13 isotope and more of the lighter carbon-12 isotope. By examining the isotopic composition of potential source materials, it might be possible to identify the sources of organic matter to river bed sediments.

Description of Study Area

The Tualatin River basin is a 712 mi² watershed in northwestern Oregon. Encompassing most of Washington County and parts of Multnomah, Clackamas, and several other counties, the basin includes the western part of the Portland metropolitan area and was home to approximately 450,000 people at the time of this study (U.S. Census Bureau, 2000). The river begins in the forested Coast Range mountains to the west, meanders through a valley bottom farmed for a wide variety of agricultural products, and skirts the southern boundary of the urban area before joining the Willamette River upstream (south) of Portland (fig. 1).

Five major tributaries enter the Tualatin River, each of which has distinct characteristics. Scoggins Creek has most of its drainage in the Coast Range and contains the basin’s only reservoir, Henry Hagg Lake, which stores water for irrigation, municipal use, and flow augmentation during summer. Gales Creek also has a significant headwater area in the Coast Range and primarily is forested (70 percent). Dairy Creek is located in the northern center of the basin and has a large area of agricultural land (50 percent) in its drainage. Rock and Fanno Creeks drain the northeastern part of the basin, and contain a substantial amount of urban land use (66 and 100 percent, respectively; data from 2001 National Land Cover Database, see Homer and others, 2007).

Streamflow in the Tualatin River reflects the regional climate, with higher flow during the winter rainy season (approximately November–April) and the lowest flow during the dry summer season (May–October). Typical peak annual flows are greater than 5,000 ft³/s, whereas typical summertime low flows are less than 200 ft³/s. Summer flows are augmented to improve water quality through releases from Henry Hagg Lake and from Barney Reservoir, a smaller storage reservoir in the adjacent Trask River basin to the west. Treated wastewater from the Rock Creek and Durham advanced WWTFs also add a substantial amount of flow to the river. During late summer, treated wastewater and reservoir releases can account for as much as 50 to 75 percent of the flow in the river (Tualatin River Flow Management Technical Committee, 2000).

The slope, width, adjacent vegetation, and substrate of the Tualatin River have an effect on the water quality of the river. In the Coast Range, the river is steep and runs over a bedrock substrate that includes several waterfalls. It is clear, cool, and well shaded, and as a result, it is well oxygenated. Where the river meets the valley bottom upstream of Gaston, its slope decreases to just more than 1 ft/mi and it begins to suspend fine particulate material as it meanders on valley sediments. The river from Gaston to river mile (RM) 30 is about 50 ft wide and still relatively well shaded. The lack of light means that few algae grow in this reach. Near RM 30, the river transitions to a pooled reach with a slope less than 0.1 ft/mi and a width of about 150 ft, allowing solar radiation to stimulate the growth of algae. The pooled reach, with depths that often exceed 15 ft, is a depositional area that accumulates a large amount of organic-rich sediment. This reach extends downstream to RM 3.4 at the Oswego Dam, a 4-ft high concrete structure built on a shallow bedrock sill. During summer, the residence time in the pooled reach can be 10–14 days, which is sufficient time to grow a large population of phytoplankton (free-floating algae) and/or deplete a substantial amount of DO through SOD.

Purpose and Scope

This report describes the use of carbon and nitrogen stable isotope measurements and carbon/nitrogen (C/N) ratios to identify the likely sources of organic material to bed sediments of the Tualatin River. In particular, the study addresses the following questions:

• Does bed sediment have a similar isotopic composition throughout the Tualatin River?
• Is the isotopic composition of bed sediment in the Tualatin River similar to that in the tributaries?
• What are the isotopic compositions of potential source materials of bed sediment?
• Based on isotopic composition, what source materials appear to contribute most to bed sediment, and is it likely that the contribution of algae is significant?

These questions are critical to the management of water quality in the Tualatin River basin and will help to determine the priority of various source-control efforts related to the TMDLs.

The focus of this study was on bed sediment in the lower Tualatin River where decomposing organic materials on the river bed can consume large amounts of dissolved oxygen. Sediment oxygen demand tends to be most important in the pooled reach of the river between RM 30 downstream of Farmington and RM 3.4 at the Oswego Dam because the slow travel time in this reach favors the exertion of SOD. The scope of the study, however, includes bed sediment samples from farther upstream and from Tualatin River tributaries as well as a wide range of potential source materials throughout the drainage basin. Sampled source materials included soils, leaf litter and detritus, plankton, aquatic plants, WWTF effluent particulate, and suspended sediment. To examine seasonal variations and assess interannual variability, samples were collected at different times of year between late summer of 1998 and the summer of 2000.

First posted August 17, 2010