[1] Hyporheic flow can be extremely variable in space and time, and our understanding of complicated flow systems, such as exchange around small dams, has generally been limited to reach-averaged parameters or discrete point measurements. Emerging techniques are starting to fill the void between these disparate scales, increasing the utility of hyporheic research. When ambient diurnal temperature patterns are collected at high spatial resolution across vertical profiles in the streambed, the data can be applied to one-dimensional conduction-advection-dispersion models to quantitatively describe the vertical component of hyporheic flux at the same high spatial resolution. We have built on recent work by constructing custom fiber-optic distributed temperature sensors with 0.014 m spatial resolution that are robust enough to be installed by hand into the streambed, maintain high signal strength, and permit several sensors to be run in series off a single distributed temperature sensing unit. Data were collected continuously for 1 month above two beaver dams in a Wyoming stream to determine the spatial and temporal nature of vertical flux induced by the dams. Flux was organized by streambed morphology with strong, variable gradients with depth indicating a transition to horizontal flow across a spectrum of hyporheic flow paths. Several profiles showed contrasting temporal trends as discharge decreased by 45%. The high-resolution thermal sensors, combined with powerful analytical techniques, allowed a distributed quantitative description of the morphology-driven hyporheic system not previously possible.

Groundwater has a predictable thermal signature that can beused to locate discretezones of discharge to surface water. As climate warms, surface water with strong groundwater influence will provide habitat stability and refuge for thermally stressed-aquatic species, and is therefore critical to locate and protect. Alternatively, these discrete seepage locations may serve as potential point sources of contaminants from polluted aquifers. This study compares two increasingly common heat tracingmethods to locate discrete groundwater discharge: directcontact measurements made withfiber-optic distributed temperature sensing (FO-DTS) and remote sensing measurements collected with thermal infrared (TIR) cameras. FO-DTS is used to make high spatial resolution (typically m)thermal measurements through time within the water column using temperature-sensitive cables. The spatial-temporal data can be analyzed with statistical measures to reveal zones of groundwater influence, however, the personnelrequirements, time to install, and time to georeferencethe cables can be burdensome, and the control units need constant calibration. In contrast, TIR data collection, either from handheld, airborne, or satellite platforms, can quickly capture point-in-time evaluations of groundwater seepage zones across large scales. However the remotenature of TIR measurementsmeans they can be adversely influenced by a number of environmental and physical factors, and the measurements are limited to the surface "skin" temperature of water features.We present case studies from a range of lentic to lotic aquatic systemstoidentify capabilities and limitations of both technologies and highlight situations in which one or the other might be a better instrument choice for locating groundwater discharge. FO-DTS performs well in all systems across seasons, but data collection was limited spatially by practical considerations of cable installation. TIR is found to consistently locate groundwater seepage zones above and along the streambank, but submerged seepage zones are only well identified in shallow systems (e.g. <0.5 m depth) with moderate flow. Winter data collection, when groundwater is relatively warm and buoyant, increases the water surface expression of discharge zones in shallow systems.

Groundwater discharge generates streamflow and influences stream thermal regimes. However, the water quality and thermal buffering capacity of groundwater depends on the aquifer source-depth. Here, we pair multi-year air and stream temperature signals to categorize 1729 sites across the continental United States as having major dam influence, shallow or deep groundwater signatures, or lack of pronounced groundwater (atmospheric) signatures. Approximately 40% of non-dam stream sites have substantial groundwater contributions as indicated by characteristic paired air and stream temperature signal metrics. Streams with shallow groundwater signatures account for half of all groundwater signature sites and show reduced baseflow and a higher proportion of warming trends compared to sites with deep groundwater signatures. These findings align with theory that shallow groundwater is more vulnerable to temperature increase and depletion. Streams with atmospheric signatures tend to drain watersheds with low slope and greater human disturbance, indicating reduced stream-groundwater connectivity in populated valley settings.

The retention capacity for biologically available nitrogen within streams can be influenced by dynamic hyporheic zone exchange, a process that may act as either a net source or net sink of dissolved nitrogen. Over 5 weeks, nine vertical profiles of streambed chemistry (NO 3 À and NH 4 + ) were collected above two beaver dams along with continuous high-resolution vertical hyporheic flux data. The results indicate a non-linear relation of net NO 3 À production followed by net uptake in the hyporheic zone as a function of residence time. This Lagrangian-based relation is consistent through time and across varied morphology (bars, pools, glides) above the dams, even though biogeochemical and environmental factors varied. The empirical continuum between net NO 3 À production and uptake and residence time is useful for identifying two crucial residence time thresholds: the transition to anaerobic respiration, which corresponds to the time of peak net nitrate production, and the net sink threshold, which is defined by a net uptake in NO 3 À relative to streamwater. Short-term hyporheic residence time variability at specific locations creates hot moments of net production and uptake, enhancing NO 3 À production as residence times approach the anaerobic threshold, and changing zones of net NO 3 À production to uptake as residence times increase past the net sink threshold. The anaerobic and net sink thresholds for beaver-influenced streambed morphology occur at much shorter residence times (1.3 h and 2.3 h, respectively) compared to other documented hyporheic systems, and the net sink threshold compares favorably to the lower boundary of the anaerobic threshold determined for this system with the new oxygen Damkohler number. The consistency of the residence time threshold values of NO 3 À cycling in this study, despite environmental variability and disparate morphology, indicates that NO 3 À hot moment dynamics are primarily driven by changes in physical hydrology and associated residence times. À production followed by net NO 3 À reduction with residence time yielding the increase/decrease in R N observed in the vertical, particularly at Dam 2 3748 M. A. BRIGGS ET AL.