Introduction

During summer 2001, unusually dense and extensive patches of rooted aquatic plants (macrophytes) were observed in the lower Yakima River in south-central Washington near Kiona at river mile (RM) 30 by various agency personnel and local residents. By 2003, excessive plant growth produced nuisance conditions in the lower river near Kiona that impaired recreational use and was suspected of causing conditions (low dissolved oxygen concentrations and high pH) that might adversely affect fish species in the river. In response to these concerns, the USGS and the South Yakima Conservation District conducted an assessment of eutrophication (nutrient enrichment) in the lower 116 mi of the Yakima River during the 2004–07 irrigation seasons. This study evaluated the water-quality affects from algae and macrophytes in the Yakima River, and the results gained can be used by water-quality managers and other stakeholders in the Yakima basin during water-quality improvement efforts, such as Total Maximum Daily Load (TMDL) development and implementation.

Study Objectives

This report presents the results of a four year cooperative study of nutrient enrichment in the lower Yakima River basin. The primary objectives of the study were to (1) characterize distinct reaches in the lower 116 mi of the Yakima River based on aquatic plant conditions, geomorphology, and habitat, (2) characterize nutrient and suspended-sediment conditions and calculate a load balance on nutrients and suspended sediment, including major tributaries, drains, and wastewater treatment plants to identify the major sources of nutrients, (3) characterize the extent and severity of exceedences of the Washington State water-quality standards for water temperature, dissolved oxygen, and pH, (4) determine the important factors related to the spatial and temporal patterns in aquatic plant and water-quality conditions, and (5) compare current aquatic plant and water-quality conditions to historical conditions.

Description of the Lower Yakima River Basin

The Yakima River drains a 6,155 mi² basin on the east side of the Cascade Range in south-central Washington (fig. 1). The mean annual precipitation ranges from about 140 in. in the higher mountains to less than 10 in. in the lower basin and, because only 20 to 40 percent of the annual precipitation occurs during the growing season between March and October, most crops need to be irrigated. The area is one of the most intensively irrigated areas in the United States, and surface-water diversions for irrigation are equivalent to about 60 percent of the annual streamflow for the basin (Morace and others, 1999). A major Bureau of Reclamation irrigation project includes 6 large storage reservoirs in the northwestern part of the basin that were constructed between 1908 and 1933 and 14 major diversions from the main-stem Yakima River that feed 6 major irrigation-district projects and numerous small irrigation systems (Rinella and others, 1992).

The lower Yakima River basin (fig. 2) encompasses 2,500 mi² and is separated from the upper basin by a natural break in Ahtanum Ridge called Union Gap, located at RM 106. The landscape of the lower basin reflects the extent of irrigated agriculture—irrigated crops dominate near the lowlands and small hills close to the river, and sagebrush and dryland grasses dominate in the uplands. Irrigated crops include fruits, grapes, and specialty crops such as hops and mint (Fuhrer and others, 2004). The dairy and beef industries have rapidly expanded in the lower basin since the 1990s (Fuhrer and others, 2004). The city of Yakima, located immediately upstream of Union Gap, is by far the largest city in the basin with a population of about 75,000, but the lower basin also includes many towns and small cities that depend on the agricultural economy for their livelihood. The Yakama Nation is in the southwest part of the basin and extends from the Yakima River to the crest of the Cascade Range.

The lower Yakima River is a highly managed system operated to meet diverse, often competing needs, including irrigation, fish habitat, recreation, flood control, and power generation. Flow varies widely in the river, diversions, and return drains from month to month and year to year. During the irrigation season from March through October this intensive management results in flows that are substantially different than those that would occur naturally in an unmanaged, snowmelt-dominated river in a semiarid climate. Peak flows in the lower Yakima River change abruptly during the irrigation season between reaches as water is diverted and returned to the river (fig. 3). Irrigation diversions begin in March, when snowmelt from the Cascades is typically the major source of water to the river. The snowmelt period generally ends in June when the stored water in reservoirs is released. Reservoir releases continue through the end of the irrigation season in late October.

Prior to the development of major irrigation projects in the Yakima River basin in the early 1900s, the Yakima River was one of the largest anadromous fish producers in the Columbia River basin. Much of the decrease in fish population in the 20^th century has been attributed to fish-passage problems and habitat restrictions associated with irrigation development in the basin, overfishing in the Pacific Ocean, Columbia and Yakima Rivers, and hydropower development on the Columbia River. Since 1980, however, improvements have been made in conditions for spawning, rearing, downstream migration of juvenile fish, and the upstream migration of adults in the Yakima River (Rinella and others, 1992).

Although the lower Yakima River is used primarily as a migration corridor by salmonids (steelhead, spring Chinook, and coho salmon) to access spawning areas in the upper river and tributaries, fall Chinook spawning grounds mostly are located within the lower river. Steelhead populations in the Yakima River basin are included in the Mid-Columbia Distinct Population Segment, and are listed as threatened under the Endangered Species Act (National Marine Fisheries Service, 2008). Based on known life histories of the fish species that pass through the lower Yakima River, the critical period for anadromous fish is during spring and autumn—ending around mid-June for the spring out-migration and starting in mid-September for the autumn up-migration. Pacific lampreys spawn during spring and early summer months and have a migration period from March through October. Other native fish, such as those in the nonanadromous lamprey, minnow, sucker, and sculpin families, also reside in the lower Yakima River (Richard Visser, Washington Department of Fish and Wildlife, written commun., 2008).

Eutrophication

One consequence related to the nutrient enrichment, or eutrophication, of streams is the increased growth of aquatic plants (algae and macrophytes) to levels that cause degraded water quality and interfere with uses such as fisheries, recreation, and agriculture. Possible secondary effects of the excessive growth of aquatic plants are low dissolved oxygen from respiration and high pH from photosynthesis that can negatively affect aquatic life. In addition, aquatic plants can provide habitat for opportunistic predators (for example, bass) that may prey on smaller salmonids.

Nutrient Sources, Transport, and Cycling

Although some nutrients enter the lower Yakima River directly from wastewater treatment plants, most nutrients come from diffuse sources such as atmospheric deposition, runoff and leaching from fertilizer application and animal waste, urban runoff, decay of vegetation, and septic system effluent. Nutrients from diffuse sources are delivered to the Yakima River through tributaries, irrigation return flows, overland runoff, and ground water (fig. 4).

The complex factors controlling the delivery of nutrients to the lower Yakima River operate at different time scales and are influenced by climate, land cover and land use characteristics, and soil properties (Shepherd and others, 1999). Whereas nutrients from wastewater treatment plants are delivered immediately to the river and those from overland runoff are delivered to the river within hours or days of a precipitation event or irrigation application, the time required for nutrients to travel to the river through ground water ranges from months to decades. The transport and transformation of nutrients depend on a complex system of biotic and abiotic processes that include nitrogen fixation by bacteria, uptake and release by aquatic organisms, adsorption on soil particles, and deposition and entrainment (Webster and Swank, 1985).

Types of Aquatic Plants Algae are photosynthetic organisms that contain chlorophyll and are important components of stream ecosystems because they convert sunlight into energy and are the beginning of the food chain for many stream organisms. Stream algae include free-floating phytoplankton and attached algae. Periphytic algae (for example, diatoms, filamentous green algae, and blue-green algae), also called benthic algae, are attached to rocks, whereas epiphytic algae are attached to other aquatic plants. Because algae lack true stems, roots, and leaves, they must obtain nutrients directly from the surrounding water. Macrophytes are vascular aquatic plants (some species have roots and some do not) that can be an important part of aquatic ecosystems because they provide habitat and food for diverse aquatic organisms and increase ecosystem complexity (Bowden and others, 2006).

Factors Related to the Distribution and Abundance of Aquatic Plants

Algae

The level of algal biomass depends on the physical, chemical, and biological characteristics of a stream, including water velocity, water temperature, light availability, and nutrient concentrations (Biggs and Close, 1989; Steinman, 1996). Hydrologic conditions also may affect algal biomass through physical scouring, especially during high flow events, and grazing by benthic invertebrates and herbivorous fish also can reduce algal biomass (Steinman, 1996). A summary of the potential factors affecting algal biomass given adequate nutrient concentrations and nontoxic conditions is shown in table 1.

Macrophytes

Light availability, rather than nutrient availability, is a common factor limiting macrophyte growth (Madsen and others, 2001)—turbidity levels, phytoplankton abundance, and water depth all affect light availability (Barko and others, 1986; U.S. Environmental Protection Agency, 2000a). Rooted macrophytes obtain nitrogen and phosphorus either through roots in the bed sediment or through shoots in the water column, and macrophytes with extensive root systems are able to meet their nutrient needs predominantly from the bed sediment (Carignan, 1982; Chambers and Prepas, 1989; Barko and others, 1991).

Previous Studies

The Yakima River is generally well-studied compared to other moderately sized rivers in Washington State, but no detailed studies addressing eutrophication processes have been done. USGS studies of the Yakima River have documented the spatial and temporal variation in nitrogen and phosphorus (Morace and others, 1999), analyzed trends in the concentrations of nutrients and suspended sediment (Ebbert and others, 2003), and described the distribution of fish, benthic invertebrate, and algal communities (Cuffney and others, 1997). Regional and national studies that have included data collected in the Yakima River basin have analyzed the relation between nutrient concentrations and land use (Mueller and Spahr, 2006) and the trends in nutrient concentrations (Wise and others, 2007). The Washington State Department of Ecology (WA DOE) modeled the potential effect of relocating a major diversion point near RM 37, including changes in dissolved oxygen, pH, and temperature (Carrol and Joy, 2002).