|
|
Three sediment oxygen demand (SOD) measurement chambers were
deployed in the Tualatin River near Tigard, Oregon, at river mile 10 in
August 2000. SOD rates were calculated for three different circulation
velocities during each chamber deployment. The SOD rate at each
velocity was calculated from a graph of dissolved oxygen concentration
versus elapsed time. An acoustic doppler current profiler (ADCP) was
used to measure stream discharge and near-bottom water velocities in
the Tualatin at river mile 10 and at two upstream locations. Measured
river and chamber velocities were similar, indicating that results from
the chambers were representative of instream effects.
At low to moderate chamber circulation velocities (less than about 7.5
centimeters per second), the measured SOD rate appeared to be only
slightly affected by the circulation velocity, indicating that the
measured rates reflect the rate of oxygen utilization by chemical and
biological reactions in the sediment rather than the rate of physical
transport of oxygen to the sediment-water interface. Above about 7.5
centimeters per second, however, the measured oxygen depletion rate was
affected by the circulation velocity, as sufficient energy was
generated within the chamber to resuspend bottom sediment, as evidenced
by increased turbidity. The resuspended sediment particles contributed
to the measured oxygen loss rate by increasing the surface area of
decomposing material in contact with the water column, resulting in a
measured SOD rate that was anomalously high. Two different alignments
of the chamber circulation diffusers were tested. With both diffuser
alignments, SOD rates were similar at circulation velocities low enough
to avoid sediment resuspension.
The same resuspension effect probably exists in the Tualatin River
during storm-runoff events following prolonged periods of low flow,
when increased stream velocity may result in the resuspension of bottom
sediments. The resuspension causes increased turbidity and increased
oxygen demand, resulting in lower instream dissolved oxygen
concentrations.
|
