Study Area and Site Selection
We quantified attributes and drivers of stream drying in the Ouachita highlands of southeastern Oklahoma. We chose this ecoregion because the hydrology remains relatively unaltered (Fox and Magoulick 2019). Ouachita highland streams are generally high gradient and clear, with rocky substrates and riffle-pool morphology (Geise et al. 1987). Within the Ouachita mountains, more than 58% of total stream length is made up of headwater streams and 17-81% of total stream length is intermittent, as streams may be simultaneously classified as both intermittent and headwater (Nadeau and Rains 2007). Hydrology is driven by precipitation runoff with little groundwater influx and streams typically dry in the summer (Giese et al. 1987; Williams et al. 2002; Leasure et al. 2016). The Oklahoma Mesonet site located in Mt. Herman reports 1310.64 mm of average annual precipitation over the 30-year period from 1991-2020 (Oklahoma Climatological Survey, Mt. Herman, OK, 2023).
Recent flow classification schemes described headwater streams in the Ouachita Mountains as being dominated by intermittent flashy and intermittent runoff regimes, where stream flows closely mirror precipitation events and exhibit regular no-flow days, thus allowing for predictions of local flow characteristics (Leasure et al. 2016; Fox and Magoulick 2019). Intermittent flashy flow regimes are predicted to exhibit drying for 1 to 3 months annually and occur in small drainage areas, while intermittent runoff regimes are predicted to have 14-50 no-flow days annually and occur in slightly larger drainage areas (70-622 km2) (Leasure et al. 2016). Streams generally exhibit drying patterns from early summer through the fall and have a period of elevated baseflow during the wetter winter months (Eng et al. 2015).
We selected 10 sites on 10 unique stream segments within the Glover River basin, a north-south free-flowing (undammed) tributary of the Little River in McCurtain County, Oklahoma (Figure 1). Each site consisted of a representative riffle and pool (in sequence). We selected sites along a hypothesized intermittency gradient based in part on the flow regime classifications of Leasure et al. (2016), watershed area, and stream slope to capture a range of intermittency. The USGS maintains a stream gauging station on the mainstem Glover River (no. 07337900) downstream of the study area, and there are no other gauges within the basin.