Water quality in three potential drought refuges in an arid-land river: assessing habitat suitability for at-risk fish species

Drought is a common disturbance in arid-land streams and rivers. The survival of aquatic species depends on access to refuge habitats where water quality remains high. Over the past century, modified flow regimes and altered watershed and instream characteristics have led to the extinction and endangerment of numerous fish species endemic to the southwestern United States. We assessed the water quality of potential drought refuges in the Middle Rio Grande (MRG), with an emphasis on suitability for the endangered Rio Grande Silvery Minnow (RGSM). We examined three types of potential drought refuges: three agricultural return drain outfalls; three isolated pools that remained during streamflow intermittency; and a reach with perennial flow below an agricultural diversion dam. All potential refuges are known to contain RGSM and other fishes. Two out of three drain outfalls, one out of three isolated pools, and three out of ten kilometers of perennially wetted stream below a dam met basic water quality criteria necessary to support RGSM populations. These findings suggest that refuge habitability is context dependent, that generalizations regarding the suitability of a specific refuge type should be avoided, and that careful assessment is required to determine if a specific location will support fish assemblages. Although some areas may contain water, they may represent ecological traps if fish are exposed to poor water quality conditions compared to other potential refuge habitats.