SEASONAL AND SPATIAL CHANGES IN THE DISSOLVED LOAD OF THE SPOKANE RIVER (WASHINGTON STATE, USA)

The Spokane River, which flows ~110 miles (180 km) from Lake Coeur d’Alene in northern Idaho through eastern Washington to the Columbia River, has been contaminated by historical mining practices upstream of Lake Coeur d’Alene as well as urban inputs such as stormwater and sewer overflow. As a result, people are advised to limit both their consumption of fish from some stretches of the river as well as their contact with shoreline sediments due to high concentrations of metals (e.g., lead).

We investigated the seasonal and spatial changes in the dissolved load along the upper 30-mile stretch of the river (from Lake Coeur d’Alene to the city of Spokane). We collected water samples every two to three weeks for over a year from six locations. Samples were analyzed for major and minor cations and heavy metals (cadmium, lead, zinc). Water temperature and pH were measured in situ, and alkalinity was determined in the lab by titration. Stream discharge data were collected from the USGS database.

Spatial variation in hydrochemistry was observed between losing and gaining reaches of the river; for example, calcium and alkalinity concentrations were consistently higher in the gaining reaches. Seasonal changes in the dissolved load were a function of stream discharge. Concentrations of alkalinity, calcium, magnesium, and sodium were diluted during snowmelt and rain events. However, zinc concentrations were positively correlated with stream discharge. Possible sources of zinc are storm water and river sediments contaminated from upstream mining practices. Understanding the sources and timing of loading of heavy metals to the river, whether anthropogenic or natural, is important for watershed management.