Water Stewardship Information Sources
| ID | 1030 |
|---|---|
| Citation | Josephy, A. 2011. Effects of Zosel Dam water regulation on Osoyoos Lake water quality (Study 4). Prepared for International Joint Commission. |
| Organization | International Joint Commission |
| URL | http://ijc.org/rel/boards/osoyoos/Study%204%20Final%2020110506.pdf |
| Abstract/Description or Keywords | The State of Washington Water Research Center was tasked by the Washington State Department of Ecology to assess the effects of Zosel Dam on water quality in Osoyoos Lake. The purpose of this Study 4 is to inform the development of a new Order of Approval, which will prescribe dam operations starting in 2013. Zosel Dam is located on the southern end of Osoyoos Lake and is capable of storing around 20,000 acre-feet (24 million m3 ) by raising the lake's elevation from around 909 feet, the minimum elevation of the dam, to 912.5 feet, the maximum elevation of the dam. The dam plays little role in regulating flow rates through the lake - its main role is to maintain summer and fall lake elevations around 911.5 feet during most non-drought "normal" years, around 912.5 feet during most "drought" years, and around 909.5 feet in most winters. Primary uses of the lake include recreation, irrigation, drinking water, and habitat for aquatic life, including Kokanee Salmon (Oncorhynchus nerka). Nutrient control efforts since the early 1970s have resulted in an order of magnitude decrease in phosphorus loading to lakes in the Okanagan River Basin, including Osoyoos Lake. Decreases in external phosphorus loading from point sources to Osoyoos Lake has led to lower phosphorus levels in the lake. Between 1999 and 2009, Secchi depths in the lake have generally ranged from two to six meters, with depths tending to be greatest in the north basin (3-6 m) and lowest in the south basin (2-4 m). Chlorophyll a levels range from 0-0.015 mg/L, with the lowest concentrations typically measured in the North Basin. Total phosphorus (TP) levels are generally below 0.02 mg-P/L and are commonly below water quality objectives of 0.015 mg-P/L to protect recreation and aesthetic uses. The lake's trophic status has improved from eutrophic in the 1970s to mesotrophic presently. All three basins exhibit declines in bottom water dissolved oxygen levels after thermal stratification. Bottom waters in the central and south basins are void of oxygen for much of the late summer and fall. Data analyses in this study suggest that nutrient and phytoplankton levels in the lake are related to, and partly controlled by, inflow to Osoyoos Lake from the Okanagan River. Nutrient and phytoplankton levels were slightly higher during normal flow versus drought years having typically lower inflows. For example, summer TP in normal versus drought years averaged 0.020 versus 0.016 mg-P/L for the north basin, 0.051 versus 0.033 mg-P/L for the central basin, and 0.044 versus 0.025 mg-P/L for the south basin. Seasonal inflows also tended to positively correlate with average summer TP and chlorophyll a. Strong correlations were found between spring inflows and summer TP and chlorophyll a in the north basin, and between spring and summer flows and TP in the central and south basins. The strongest relationships were found in the central basin, where the r2 for the linear correlation between average spring and summer inflows and TP was around 0.5. Zosel Dam exerts no control on lake inflow and only effects lake elevation and water depth minimally from year to year (i.e., differences of a few feet). Therefore, we are unable to suggest changes in dam operation that would directly and knowingly affect water quality in Osoyoos Lake. Rather than relying on changes in dam operations to impact water quality, we suggest that lake managers focus on the continued control of nutrient loading to the lake. We also identified internal nutrient loading as a potentially important controller of summer productivity, particularly in the central and south basins. As a result, lake managers should evaluate the significance of internal nutrient loading on water quality in Osoyoos Lake. The impact of summer bottom water anoxia on cold water fisheries also warrants further examination. One potential management strategy to ameliorate both internal nutrient loading and bottom water anoxia is lake oxygenation. Lake oxygenation is an engineered system that uses pure oxygen gas to enhance the oxygen content of bottom waters. These systems have been successfully operated elsewhere in the region and country, and could be a component of further improving water quality in Osoyoos Lake. dissolved oxygen, temperature, salmonids, aquatic habitat, eutrophication |
| Information Type | report |
| Regional Watershed | Okanagan |
| Sub-watershed if known | Osoyoos Lake |
| Aquifer # | |
| Comments | |
| Project status | complete |
| Contact Name | |
| Contact Email |