A BIOLOGICAL CAUSE FOR LARGE SUMMER DIEL STREAMFLOW CYCLES, UPPER BIG HOLE RIVER, SOUTHWESTERN MONTANA

During summer, amplitudes of diel streamflow cycles in the upper Big Hole River can vary substantially from year to year at a site 15 miles downstream from Wisdom, Montana. For example, flows during mid-August 2005 ranged from 55-101 ft³/s in the late afternoon to 141-210 ft³/s before dawn, with daily increases of as much as 156% from the minimum. In contrast, daily increases for the same period during 2007 were only 12-24%. Possible causes of the interannual variation in diel flow cycles were studied by review of historical data as well as synoptic sampling of tributaries upstream from the site and repeated measurement of streamflow and water quality during diel sampling of the mainstem on September 7-8, 2005.

Evapotranspiration (ET) by riparian vegetation is not the main cause of the diel flow cycles in this reach of the Big Hole River because (1) estimates of potential water uptake by riparian vegetation are much lower that the diel flow fluctuations, (2) diel cycles in concentrations of conservative ions that often accompany ET-induced diel flow cycles and that would be expected based on calculated composition of groundwater inflow are absent, and (3) the magnitude of the diel flow cycles varies substantially from year to year.

Alternatively, the diel flow cycles are thought to be driven by diel changes in biological activity of periphyton and macrophytes that “puff up,” perhaps related to O₂ production, while actively photosynthesizing during the day to “dam” the river, and then relax during the night to release the pooled water. This hypothesis is reasonable because (1) diel flow cycles are reduced in years when unusually high spring runoff presumably scoured the streambed and removed much of the aquatic vegetation, (2) river stage increased while measured flow decreased during mid-afternoon of the diel sampling episode, and (3) the magnitude of the diel flow cycles during August 2005 correlated more closely with daily mean water temperature (R² = 0.73) than with daily mean air temperature (R² = 0.33), indicating that the effect of temperature on biological activity of aquatic vegetation may have been more pronounced than the effect on water consumption by riparian ET. This proposed biological damming phenomenon may affect other streams, which, like the Big Hole River, are wide, shallow, and biologically productive.