USGS:  Sediment Oxygen Demand in Lake Ewauna and the Klamath River, Oregon, June 2003

The Klamath River, looking downstream, just below Upper Klamath Lake in Oregon. In summer, water rich in decomposing organic material (primarily from algae blooms) flows into the Klamath River from Upper Klamath Lake, causing a large oxygen demand in the sediment and water column, depleting oxygen in the river.

Abstract

Sediment oxygen demand (SOD) is the rate at which dissolved oxygen is removed from the water column during the decomposition of organic matter in streambed or lakebed sediments. In lakes and slow moving rivers, or rivers with high levels of organic matter in the bed sediment, SOD can be a major cause of low dissolved oxygen (DO) concentrations in the water column. Low DO concentrations can be detrimental to fish and other aquatic life in a stream or lake.

In June 2003, replicate SOD measurements were made at one site in Lake Ewauna and three sites in the Klamath River above Keno Dam. Individual measurements of SOD₂₀ rates(temperature corrected to 20 degrees Celsius) ranged from 0.3 to 2.9 g O₂/m²/day (grams of oxygen per square meter per day), with a median value of 1.8 g O₂/m²/day (n=22). The variability of individual SOD measurements at a site was equal to or greater than the variability of SOD rates among the four sites. Consequently, the overall median SOD for the entire study reach should be used in water-quality models. SOD values measured in this study are about 10 times smaller than estimates made in this study area by the Oregon Department of Environmental Quality in 1994. The rates measured in the Klamath River are similar to SOD rates measured several miles upstream in Upper Klamath Lake in 1999. These two findings indicate that the unusually high SOD rates in the Klamath River result from causes other than the presence of large amounts of woody debris in bottom sediments from past and ongoing sawmill operations, which has been suggested by other investigators.

An areal rate of water-column oxygen demand was estimated from control chambers during the study. In early June 2003, the median rate was 3.8 g O₂/m²/day, which was more than double the SOD₂₀ rate. Taken together, the SOD and water-column oxygen demand can more than account for the severe hypoxia that develops in this reach of the Klamath River from July into October. The largest source of labile organic matter contributing to this hypoxia likely comes from algal blooms in Upper Klamath Lake.

Bed-sediment samples were collected and correlations were considered between SOD₂₀ rate and percentage of organic material, and SOD₂₀ rate and percentage of sediment finer than 63 microns. Based on these correlations, it does not appear that SOD variability can be readily determined from these two sediment characteristics.