Summary Global change is impacting the forests of the western United States through rising temperatures, earlier snowmelt, more rain and less snow, greater vapour pressure deficits in spring and autumn, forest dieback and increasing forest fire frequency and severity. A catastrophic forest fire (Las Conchas fire) occurred in central NM, USA, in 2011 burning c. 634 km2 with c. 46% of the fire being of severe or moderate intensity. National Oceanic and Atmospheric Administration (NOAA) next‐generation radar data (NEXRAD) were used to link precipitation events occurring in the burn scar to extreme water quality excursions observed in the Rio Grande. At four sites, in situ sensors captured the response of water temperature, specific conductance, pH, turbidity and dissolved oxygen to flood events following the fire. Runoff from burn scars caused turbidity peaks (to 2500 NTU), dissolved oxygen sags (to 0.0 mg L‐1), pH sags (up to 0.75 units) and conductivity changes (both increases and decreases). These water quality excursions extended at least 50 km downstream, with significant implications for the ecosystem health of this crucial river that supplies water to cities and agriculture. Sudden, dramatic changes to forested catchments from severe forest fires and forest dieback are very likely to be among the strongest impacts of global change on stream and river ecosystems throughout the western United States.

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Wildfires are increasing globally in frequency, severity, and extent, but their impact on fluvial networks, and the resources they provide, remains unclear. We combine remote sensing of burn perimeter and severity, in-situ water quality monitoring, and longitudinal modeling to create the first large-scale, long-term estimates of stream+river length impacted by wildfire for the western US. We find that wildfires directly impact ~6% of the total stream+river length between 1984 and 2014, increasing at a rate of 342 km/year. When longitudinal propagation of water quality impacts is included, we estimate that wildfires affect ~11% of the total stream+river length. Our results indicate that wildfire activity is one of the largest drivers of aquatic impairment, though it is not routinely reported by regulatory agencies, as wildfire impacts on fluvial networks remain unconstrained. We identify key actions to address this knowledge gap and better understand the growing threat to fluvial networks, water security, and public health risks.

Streamflow intermittency can reshape fish assemblages and present challenges to recovery of imperiled species. During streamflow intermittency, fish can be subjected to a variety of stressors, including exposure to crowding, high water temperatures, and low dissolved oxygen, resulting in sublethal effects or mortality. Rescue of fishes is often used as a conservation tool to mitigate the negative impacts of streamflow intermittency. The effectiveness of such actions is rarely evaluated. Here, we use multi-year water quality data collected from isolated pools during rescue of Rio Grande silvery minnow Hybognathus amarus, an endangered minnow. We examined seasonal and diel water quality patterns to determine if fishes are exposed to sublethal and critical water temperatures or dissolved oxygen concentrations during streamflow intermittency. Further, we determined survival of rescued Rio Grande silvery minnow for 3–5 weeks post-rescue. We found that isolated pool temperatures were much warmer (>40 °C in some pools) compared to upstream perennial flows, and had larger diel fluctuations, >10 °C compared to ~5 °C, and many pools had critically low dissolved oxygen concentrations. Survival of fish rescued from isolated pools during warmer months was <10%. Reactive conservation actions such as fish rescue are often costly, and in the case of Rio Grande silvery minnow, likely ineffective. Effective conservation of fishes threatened by streamflow intermittency should focus on restoring natural flow regimes that restore the natural processes under which fishes evolved.

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Seines are a common sampling gear for monitoring small-bodied fishes in wadeable, sand-bed rivers. However, sampling effort within a site can vary among time and space, even within the same monitoring program. Determining and standardizing an adequate or optimal amount of effort within a site is an important step for designing fisheries monitoring programs. If budgets are limited, sampling intensity at a site is often a trade-off between less intense sampling at a greater number of locations or more intense sampling at fewer locations. Inadequate sampling at survey sites could introduce variability into estimates of density, species richness, or relative abundance because of within-site variability. We evaluated how variability of mean catch-rates of five species of small-bodied fishes changed with increasing effort at a fixed number of sites, and how variability changed as more sites were sampled. Focal species were Rio Grande silvery minnow, flathead chub, red shiner, fathead minnow, and western mosquitofish. We sampled 20 sites with 20 hauls in 200 m, 40 hauls in 200 m, and 40 hauls in 400 m. We observed little decrease in variability around expected catch-per-unit-effort [Ê CPUE ð Þ] when 40 seine hauls were performed per site compared to 20 hauls per site, regardless of site length or species, though a 25% increase in labour was required. However, we observed considerable variability aroundÊ CPUE ð Þwhen ≤5 seine hauls were performed at each site. We recommend a minimum of 10 seine hauls per 200 m site. Our results provide guidance and starting point for monitoring programs that use seining to collect information on small fishes.

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.