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Stream drying is happening globally, with important ecological and social consequences. Most examples of stream drying come from systems influenced by dam operations or those with highly exploited aquifers. Stream drying is also thought to be driven by anthropogenic climate change; however, examples are surprisingly limited. We explored flow trends from the five recognized Mediterranean‐climate regions of the world with a focus on unregulated (non‐dammed or non‐diverted) streams with long‐term gauge records. We found consistent evidence of decreasing discharge trends, increasing zero‐flow days, and steeper downward discharge trends in smaller basins. Beyond directional trends, many systems have recently undergone shifts in flow state, including some streams that have transitioned from perennial to intermittent flow states. Our analyses provide evidence of stream drying consistent with climate change but also highlight knowledge gaps and challenges in empirically and statistically documenting flow regime shifts. We discuss the myriad consequences of losing flow and propose strategies for improving detection of and adapting to flow change. 

Abstract Studies of stream macroinvertebrates traditionally use sampling methods that target benthic habitats. These methods could underestimate biodiversity if important assemblage components exist outside of the benthic zone. To test the efficacy of different sampling methods, we collected paired reach‐wide benthic and edge samples from up to 10 study reaches in nine basins spanning an aridity gradient across the United States. Edge sampling targeted riparian‐adjacent microhabitats not typically sampled, including submerged vegetation, roots, and overhanging banks. We compared observed richness, asymptotic richness, and assemblage dissimilarity between benthic samples alone and different combinations of benthic and edge samples to determine the magnitude of increased diversity and assemblage dissimilarity values with the addition of edge sampling. We also examined how differences in richness and assemblage composition varied across an aridity gradient. The addition of edge sampling significantly increased observed richness (median increase = 29%) and asymptotic richness (median increase = 173%). Similarly, median Bray–Curtis dissimilarity values increased by as much as 0.178 when benthic and edge samples were combined. Differences in richness metrics were generally higher in arid basins, but assemblage dissimilarity either increased or decreased across the aridity gradient depending on how benthic and edge samples were combined. Our results suggest that studies that do not sample stream edges may significantly underestimate reach diversity and misrepresent assemblage compositions, with effects that can vary across climates. We urge researchers to carefully consider sampling methods in field studies spanning climatic zones and the comparability of existing data sets when conducting data synthesis studies. 

Freshwater species face numerous threats across the globe, including urbanization. Within cities in regions with drier climates, dewatering and channelization of rivers commonly occur and reduce or eliminate freshwater biodiversity. The discharge of effluent (treated wastewater) has been used to restore flow in some of these rivers, but our knowledge is negligible about how ecological communities develop and change in these unique but increasingly common ecosystems. In this study, we quantified aquatic macroinvertebrate community development in the Santa Cruz River (Arizona, U.S.A.), where effluent‐restored flow more than 100 years after the river dried up. We tracked community development over a 2‐year period in reaches where flow had been restored and compared those findings with data from a reference reach. Our study period also encompassed a massive disturbance where effluent releases temporarily ceased and sediment was dredged from the channel, allowing us to quantify the impacts of urban channel maintenance activities on recovering communities. Macroinvertebrate colonization was rapid following the initial flow restoration and channel dredging, with density and species richness values reaching or exceeding those of the reference reach within a few months, but community composition remained quite distinct after 2 years. Flow duration and the number of dry days in the month prior to sampling were the most influential factors in macroinvertebrate metrics. Simply adding effluent to dewatered urban rivers has the potential to restore diverse aquatic fauna, but targeted reintroductions may be needed for sensitive or dispersal‐limited taxa. 

Nonperennial rivers are a major—and growing—part of the global river network. New research and science-based policies are needed to ensure the sustainability of these long-overlooked waterways. 

Rivers that cease to flow are globally prevalent. Although many epithets have been used for these rivers, a consensus on terminology has not yet been reached. Doing so would facilitate a marked increase in interdisciplinary interest as well as critical need for clear regulations. Here we reviewed literature from Web of Science database searches of 12 epithets to learn (Objective 1—O1) if epithet topics are consistent across Web of Science categories using latent Dirichlet allocation topic modeling. We also analyzed publication rates and topics over time to (O2) assess changes in epithet use. We compiled literature definitions to (O3) identify how epithets have been delineated and, lastly, suggest universal terms and definitions. We found a lack of consensus in epithet use between and among various fields. We also found that epithet usage has changed over time, as research focus has shifted from description to modeling. We conclude that multiple epithets are redundant. We offer specific definitions for three epithets (non-perennial, intermittent, and ephemeral) to guide consensus on epithet use. Limiting the number of epithets used in non-perennial river research can facilitate more effective communication among research fields and provide clear guidelines for writing regulatory documents.