Potential Management Strategies to Improve Conditions

The complex relations between the growth of aquatic plants in the Yakima River and the key factors that control growth (such as nutrient concentrations, light availability, and the physical characteristics of the river) will need to be considered when a strategy is developed for improving water-quality conditions in the lower Yakima River. Streamflow modification and nutrient reductions are the two general approaches available to water-quality managers for reducing algal growth. In addition to these approaches, macrophyte growth can be reduced through direct management (for example, cutting and harvesting).

Streamflow Modification

For the same level of plant productivity, the dilution caused by an increase in streamflow entering the Zillah reach might result in higher dissolved oxygen concentrations and lower pH levels compared to the existing streamflow. An increase in streamflow in the Zillah reach, however, also might create additional surface area for algal growth, because of large deposits of cobble and other suitable habitats that are out of the water and not inundated with the current level of summer streamflow.

Although differences in light availability appeared to be the reason for the differences in macrophyte abundance between years in the Kiona reach, it would not be possible in a series of water-limited years for spring runoff to be great enough to create the prolonged turbid and deep conditions necessary to suppress macrophyte growth. The relatively stable substrate in much of the Kiona reach likely would not be disturbed under normal annual streamflow conditions in the lower Yakima River—no evidence was found that large areas of substrate were disturbed in the Kiona reach during this study even though the streamflow during some periods was 10,000–16,000 ft³/s.

Nutrient Reductions

The lowest nutrient concentrations of any of the three Yakima River reaches evaluated in this study were measured in the Zillah reach. The abundant algal growth and sparse macrophyte growth in the reach indicated that benthic algal metabolism was the primary reason for the exceedences of the water-quality standards for dissolved oxygen and pH. Because of the relatively low nutrient concentrations in the Zillah reach and the indication of intermittent nutrient limitation at two locations, dissolved oxygen and pH conditions might improve from reductions in nutrient concentrations in the reach. This could be accomplished by either reducing the nutrients entering the reach or increasing the amount of lower-nutrient concentration water entering the reach from upstream. Accurate predictions regarding the potential effectiveness of this approach requires information on the quantity and quality of ground water entering the river. The level of effort needed to reduce the amount of nutrients and algal growth in the river, however, will remain unknown until a better understanding of the relation between nutrient availability and primary productivity is developed.

Although the amount of nutrients in the Mabton reach could be reduced through nutrient reductions in the Zillah reach, the effect on aquatic plant growth and dissolved oxygen and pH conditions would be minimal unless nutrient loads from the agricultural return drains and other tributaries within the Mabton reach also were reduced. Even if this were to occur, the mean concentrations of dissolved nitrite plus nitrate and total phosphorus measured in the Mabton reach during this study would need to be reduced by 92 and 83 percent, respectively, to equal the suggested USEPA reference conditions for the Yakima basin.

Because almost all the nutrient load in the Kiona reach enters from the Mabton reach, substantial reductions in the Zillah and Mabton reaches will be necessary to reduce the loads of nutrients in the Kiona reach. Attempts to reduce nutrients in the river water might have no effect on plant abundance and the associated dissolved oxygen and pH problems, however, because heavily rooted aquatic plants such as water stargrass are able to obtain nutrients from either the water column or the bed sediment. The results from this study indicated that the macrophytes in the Kiona reach were not obtaining a measurable amount of their nutrients primarily from the river water, and the results from other studies have indicated that bed sediment usually provides a sufficient supply of nutrients for macrophyte growth.

Direct Management of Macrophytes

Direct management of the macrophytes (either through mechanical harvesting or some other type of control) might be the only feasible method for reducing macrophyte growth in the Kiona reach. Even a substantial reduction in macrophyte biomass, however, might not be sufficient to bring the Kiona reach into compliance with the water-quality standards for dissolved oxygen and pH. There was a 97 percent reduction in macrophyte biomass between 2005 and 2006 in the Kiona reach, to a level that most fish habitat studies have determined to be optimal for streams, but exceedences of the Washington State water-quality standards for dissolved oxygen and pH still occurred. The role of benthic and epiphytic algae (growing along the river margins and on the plants) in this reach was not assessed, but could help explain the persistent water-quality problems.