The HydroATLAS database provides a standardized compendium of descriptive hydro-environmental information for all watersheds and rivers of the world at high spatial resolution. Version 1.0 of HydroATLAS offers data for 56 variables, partitioned into 281 individual attributes and organized in six categories: hydrology; physiography; climate; land cover & use; soils & geology; and anthropogenic influences. HydroATLAS derives the hydro-environmental characteristics by aggregating and reformatting original data from well-established global digital maps, and by accumulating them along the drainage network from headwaters to ocean outlets. The attributes are linked to hierarchically nested sub-basins at multiple scales, as well as to individual river reaches, both extracted from the global HydroSHEDS database at 15 arc-second (~500 m) resolution. The sub-basin and river reach information is offered in two companion datasets: BasinATLAS and RiverATLAS. The standardized format of HydroATLAS ensures easy applicability while the inherent topological information supports basic network functionality such as identifying up- and downstream connections. HydroATLAS is fully compatible with other products of the overarching HydroSHEDS project enabling versatile hydro-ecological assessments for a broad user community.

Stream networks expand and contract through time, impacting chemical export, aquatic habitat, and water quality. Although recent advances improve prediction of the extent of the wetted channel network (L) based on discharge at the catchment outlet (Q), controls on the temporal variability of L remain poorly understood and unquantified. Here we develop a quantitative, conceptual framework to explore how flow regime and stream network hydraulic scaling factors co-determine the relative temporal variability in L (denoted here as the total wetted channel drainage density). Network hydraulic scaling determines how much L changes for a change in Q, while the flow regime describes how Q changes in time. We compiled datasets of colocated dynamic stream extent mapping and discharge to analyze all globally available empirical data using the presented framework. We found that although variability in L is universally damped relative to variability in Q (i.e., streamflow is relatively more variable in time than network extent), the relationship is elastic, meaning that for a given increase in the variability in Q, headwater catchments will experience greaterthan-proportional increases in the variability of L. Thus, under anticipated climatic shifts towards more volatile precipitation, relative variability in headwater stream network extents can be expected to increase even more than the relative variability of discharge itself. Comparison between network extents inferred from the L-Q relationship and blue lines on USGS topographic maps shows widespread underestimation of the wetted channel network by the blue line network.

Intermittent streams are common throughout the world, and are characterized as having variable cycles of wetting and flow cessation (Busch et al., 2020). Though intermittency is a natural phenomenon, projected changes in temperature and precipitation patterns are expected to prolong the dry season and increase the frequency of multi-year droughts in many parts of the world (Dai, 2011). This, in turn, is predicted to intensify the low-flow period and exacerbate the duration and severity of intermittent conditions (Grimm & Fisher, 1992;Larned et al., 2010). Changes to streamflow patterns can profoundly influence ecological functioning of streams by disrupting evolutionary cues (Heim et al., 2016), altering nutrient cycles (von Schiller et al., 2011), and increasing the risk of invasion by nonnative species (Larson et al., 2009). Many endemic and imperiled species that are adapted to natural intermittency may be placed at risk of extirpation from these shifts (Jaeger et al., 2014). Furthermore, changes to hydrological regimes may reduce the ecosystem services provided by intermittent streams, such as drinking water resources (Marshall et al., 2018) and nutrient cycling (Datry, Foulquier, et al., 2018), which may exacerbate stressors faced by human populations (Datry, Boulton, et al., 2018). Understanding the drivers of intermittent stream dynamics, as well as their sensitivity to climate variability, is critical for predicting how functioning of these systems may change under a volatile climate future.Previous studies on temperate and arid intermittent stream dynamics have shown that as the dry season progresses, the wetted channel contracts from the full extent of the geomorphic channel network in response to declining stream discharge (Godsey & Kirchner, 2014). The spatial and temporal patterns of intermittency are affected by meteorologic, geologic, and land cover conditions (Costigan et al., 2016), with topographic relief and lithology playing a particularly important role in influencing the spatial patterns of wetted channel contraction in intermittent stream networks (Jensen et al., 2017;Lovill et al., 2018;Prancevic & Kirchner, 2019). Growing insight into the mechanisms that control wetted channel dynamics has led to attempts to model and predict intermittent stream expansion and contraction (Pate et al., 2020;Ward

Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has contributed to amphibian declines globally, but drivers of outbreaks vary locally. Here, we explore the role of drought in population and host-disease dynamics of the endangered stream-breeding foothill yellow-legged frog (Rana boylii). In central California (USA) where severity of seasonal drought is increasing, we observed the non-native, Bd-tolerant and lentic-adapted North American bullfrog (Lithobates catesbeianus) extend into streams when flood disturbance was minimal. Analysis of skin swabs revealed that prevalence and load of Bd infection among bullfrogs was low.Yet, among the native frogs, prevalence and load intensified as the seasonal drought progressed and surface flow became intermittent. When temperatures decreased in autumn and frogs concentrated at a reduced number of water points, we found dozens of dead foothill yellow-legged frogs (2018)(2019). Necropsies suggested chytridiomycosis as the likely cause of death. Despite recent lethal outbreaks, foothill yellow-legged frog population abundance appeared resilient based on comparison to prior decades when no die-offs were observed. Wet-dry mapping of the stream channel and retrospective analysis of hydrologic records revealed that the native frogs spawn away from perennial pools, a behaviour that may allow them to avoid bullfrogs and predatory fish. In an ecological trade-off, tadpoles face the risk of the stream drying before metamorphosis. Fluctuations in population size thus corresponded to extremes of inter-annual variation in streamflow that limit recruitment rather than disease outbreaks. We conclude that hydrologic constraints, which climate change may exacerbate, appear to override the stressors of non-indigenous species and chytridiomycosis.

Stream networks expand and contract through time, impacting chemical export, aquatic ecosystem habitat, and water quality. Although recent advances improve prediction of the extent of the wetted channel network (L) based on discharge at the catchment outlet (Q), controls on the temporal variability of L remain poorly understood and unquantified. Here we develop a quantitative, conceptual framework to explore how flow regime and stream network hydraulic scaling factors co-determine the relative temporal variability in L. Network hydraulic scaling determines how much L changes for a change in Q, while the flow regime describes how Q changes in time. We compiled datasets of co-located dynamic stream extent mapping and discharge to analyze all globally available empirical data using the presented framework. We found that although variability in L is universally dampened relative to variability in Q (i.e., streamflow is relatively more variable in time than network extent), the relationship is elastic, meaning that for a given increase in the variability in Q, headwater catchments will experience greater-than-proportional increases in the variability of L. Thus, under anticipated climatic shifts towards more volatile precipitation, relative variability in headwater stream network extents can be expected to increase even more than the relative variability of discharge itself. Comparison between network extents inferred from the L-Q relationship and USGS topographic maps shows widespread underestimation of the wetted channel network by the mapped extent of both perennial and dynamic stream extents.