Comparison of Results to Historical Conditions

Aquatic Plants

For many years, the concentrations of phosphorus and nitrogen measured in the lower Yakima River have been high enough to support abundant growth of phytoplankton, periphyton, and macrophytes (Rinella and others, 1992), but macrophyte growth was not widespread or problematic. In the late 1980s, scattered patches of dense macrophytes were noted at one site downstream of Satus Creek near RM 70 (Rinella and others, 1992). A presence-absence survey at a limited number of locations: the Yakima River at Parker (RM 104), below Toppenish Creek near Satus (RM 80), and at Kiona, did not note any macrophytes (Cuffney and others, 1997). Other researchers, however, noted the presence of rooted aquatic plants within the lower Yakima River, especially in the vicinity of Horn Rapids Dam (RM 18) (Morace and others, 1999).

Since those studies were completed, light availability and streamflow conditions in the river have often been favorable for macrophyte growth. Macrophyte growth noticeably increased in the lower river beginning in 2001 (Richard Visser, Washington Department of Fish and Wildlife, written commun., 2002). USGS personnel noted the recent spread of “river grass” near the gaging station at Kiona in June 2001, after many years of decreasing turbidity and spring streamflow and after a spring that had the second lowest mean streamflow on record for that gaging station. USGS personnel noted that the “river grass” returned during each summer in 2002 and 2003.

USGS personnel also have observed nuisance levels of aquatic growth at Kiona in the more-distant past (fig. 29). Starting in 1933, the field notes for the gaging station showed a clear relationship between low streamflow in spring (March 1–June 30) and the mention of aquatic growth, including one reference to “river grass”. In almost all years when field personnel noted aquatic growth in the river (1939, 1941, 1942, 1944, 1945, 1977, and 1994), the mean spring streamflows were less than 10 percent of the mean spring streamflows for the station. Based on recent and historical observations at the Kiona gaging station, the macrophyte growth in the lower river appears to follow a pattern where plant growth increases during periods with low spring streamflow and decreases during periods of average or above-average spring streamflows—a pattern that also was observed during this study.

Nutrients, Suspended Sediment, and Turbidity

The median concentrations of DIN and suspended sediment at Kiona were significantly less during 2001–03 than the median concentrations during 1975–2000 (fig. 30A, C), even though the mean streamflow was significantly less during 2001–03. In contrast, the median concentration of SRP at Kiona was significantly greater during 2001–03 than the median concentration during 1975–2000 (fig. 30B). These two periods were selected for comparison because the years 1975–2000 were before the recent period of below-average spring streamflows, whereas the years 2001–03 were during the recent period of below-average spring streamflows.

These results indicate that the DIN and suspended sediment loads in the river decreased between the two periods, because concentrations would have increased if there had been no change in load. The decreased DIN load could have been due to decreases in the load from agricultural return drains during the period—the mean irrigation-season nitrate loads from Sulphur Creek Wasteway and Granger Drain were 34 percent and 24 percent less, respectively, during the 2001–04 period compared to the 1997–2000 period (Zuroske, 2006). The decreased suspended sediment load likely was due to a combination of lower sediment loading to the river from agricultural return drains and an increase in sediment deposition in the Kiona reach due to the presence of macrophytes.

A Lagrangian synoptic survey of the Yakima River for nutrients and suspended sediment was conducted by the USGS between July 26 and 29, 1988 (Rinella and others, 1992) and sampled many of the same sites that were sampled during the 2004 survey. The pattern in the longitudinal nutrient and sediment loads was similar during both years—relatively high loads upstream of the Parker diversions, decreasing substantially after the diversions, increasing steadily until Prosser Dam, decreasing after the diversion for the Chandler Canal, and increasing after the return from Chandler Canal (fig. 31).

The median spring (March 1–June 30) turbidity at Kiona was significantly less during 2004–05 compared to 1978–2000, but significantly greater during 2006–07 compared to 1978–2000 (turbidity data was not available before 1978) (fig. 32). The median summer (July 1–August 31) turbidity at Kiona was significantly less during 2004–05 and 2006–07 compared to 1978–2000.

Dissolved Oxygen and pH

The results from 3 days of continuous monitoring by WA DOE in late July 2000 at Kiona (the only continuous monitoring data available for Kiona before the start of this study) showed a daily range in dissolved oxygen concentration of about 3.7 mg/L and a minimum daily dissolved oxygen concentration of 7.5 mg/L. This was slightly below the Washington State standard of 8 mg/L, but not nearly as low as the daily minimum dissolved oxygen concentrations measured during late July 2004 and 2005, when the concentrations were consistently below 6 mg/L and often below 4 mg/L. The daily minimum dissolved oxygen concentrations in 2000 were similar to the daily minimum values measured during late July 2006, when macrophyte biomass levels were the lowest of any year during this study. The maximum daily pH level in late July 2000 (8.7 units) was greater than the Washington State standard of 8.5 units, similar to the maximum daily pH measured in late July 2004 and 2006, but less than the values measured in late July 2005 and 2007.