Landscape controls on the distribution and ecohydrology of central Oregon springs

Abstract

Small springs in semiarid landscapes are essential for maintaining aquatic biodiversity and supporting livestock grazing operations. However, little is known about controls on the distribution and physical characteristics of small springs, the aquatic species they support, or their sensitivity to disturbance. We address this information gap in the Crooked River subbasin, a tributary of the Deschutes River in Oregon. We conducted spatial analyses on 2,519 mapped springs to investigate the influence of landscape controls (precipitation and bedrock permeability) on spring density in the Crooked River subbasin and the adjacent Upper Deschutes subbasin. Spring density was highest in areas of low bedrock permeability (P < 0.0001) and high annual precipitation (P < 0.0001). We suggest that the high density of small springs on low‐permeability bedrock indicates that these springs generally have short, shallow flow paths and thus may be susceptible to forecasted climate changes. A survey of 137 springs in the Crooked River subbasin revealed the hydrogeologic setting affects spring discharge type (P = 0.017), temperature (P = 0.011), and pH (P = 0.026). We found a high frequency of anthropogenic impacts on springs: 95% of diffuse‐discharge springs and 79% of discrete‐discharge springs were disturbed by livestock grazing. Species inventories at 10 of the most intact surveyed springs confirm that small springs are biologically diverse, with 151 total species of plants and 135 total taxa of macroinvertebrates. Springs in the Crooked River subbasin are ecologically important habitats but require careful management to protect against livestock disturbance and development.

Springs ecosystem classification

“…In general support of the structural geology approach, Naghibi and Dashtpagerdi (2017) found that the best predictors of the location of 842 springs in the Khalkhal region of Iran were altitude, topographic wetness index, slope angle, and faulty density. Freed et al (2019) reported that springs density in the Crooked River sub‐basin of the Deshutes River in Oregon was highest for springs discharging from low‐permeability rock units. However, description of springs in relation to landscape scale structural geology varies among physiographic provinces and does not distinguish individual springs.…”

Groundwater, biodiversity, and the role of flow system scale

“…Reductions or changes in the volume or timing of groundwater discharge, whether from pumping or climate change, can affect GDEs in several ways. Changes in the amount and timing of groundwater discharge to lotic GDEs such as springs and rivers can lead to ecosystem drying, increased temperature and other water quality impairments, with direct impacts to GDEs (De Graaf et al, 2019; Eamus et al, 2015; Freed et al, 2019; Rohde et al, 2019). Within lentic GDEs such as fens and other groundwater‐fed wetlands, reductions in discharge or long late‐season recessions can lead to lowered water tables and organic soil oxidation, often leading to replacement of obligate groundwater‐dependent species with more generalist ones (Aldous & Bach, 2014).…”

“…This emphasis on the study and regulation of regional groundwater flow paths benefits our understanding of the ecohydrology of GDEs in low hypsographic positions. However, many small, low discharge, poorly studied GDEs are destroyed by dewatering of shallow local flow paths in ways that are not regulated and largely ignored by land and water managers (Freed et al, 2019; Sada et al, 2005; Sada & Vinyard, 2002). Conserving the full suite of biodiversity associated with groundwater discharge requires more focused attention on the smaller more local flow path associated GDEs.…”

“…In our own research across the Deschutes and Klamath Basins in central Oregon, we mapped 137 small springs (median flow: 2.6 × 10 −4 m 3 /s; Freed et al, 2019) and 70 groundwater‐supported fens (mean discharge: 3.5 × 10 −4 m 3 /s; Aldous et al, 2014; Aldous et al, 2015), all of which are supported by short flow path, low discharge local groundwater flow systems. These ecosystems are found at generally higher hypsographic positions: 66%–97% for the fens and 35%–98% for the springs.…”

Managing Groundwater to Ensure Ecosystem Function

“…Second, groundwater-dependent ecosystems are often associated with endemic and sensitive species with low resilience to impacts (Sada et al 2005;Kodric-Brown and Brown 2007;Freed et al 2019;Wolaver et al 2020). In arid regions in particular, such species may be found in only a few locations (Wolaver et al 2020).…”