[1] Over the past 3 decades, nutrient spiraling has become a unifying paradigm for stream biogeochemical research. This paper presents (1) a quantitative synthesis of the nutrient spiraling literature and (2) application of these data to elucidate trends in nutrient spiraling within stream networks. Results are based on 404 individual experiments on ammonium (NH 4 ), nitrate (NO 3 ), and phosphate (PO 4 ) from 52 published studies. Sixty-nine percent of the experiments were performed in first-and second-order streams, and 31% were performed in third-to fifth-order streams. Uptake lengths, S w , of NH 4 (median = 86 m) and PO 4 (median = 96 m) were significantly different (a = 0.05) than NO 3 (median = 236 m). Areal uptake rates of NH 4 (median = 28 mg m À2 min À1 ) were significantly different than NO 3 and PO 4 (median = 15 and 14 mg m À2 min À1 , respectively). There were significant differences among NH 4 , NO 3 , and PO 4 uptake velocity (median = 5, 1, and 2 mm min À1 , respectively). Correlation analysis results were equivocal on the effect of transient storage on nutrient spiraling. Application of these data to a stream network model showed that recycling (defined here as stream length Ä S w ) of NH 4 and NO 3 generally increased with stream order, while PO 4 recycling remained constant along a first-to fifth-order stream gradient. Within this hypothetical stream network, cumulative NH 4 uptake decreased slightly with stream order, while cumulative NO 3 and PO 4 uptake increased with stream order. These data suggest the importance of larger rivers to nutrient spiraling and the need to consider how stream networks affect nutrient flux between terrestrial and marine ecosystems.

[1] Common in-stream geomorphic structures such as debris dams and steps can drive hyporheic exchange in streams. Exchange is important for ecological stream function, and restoring function is a goal of many stream restoration projects, yet the connection between in-stream geomorphic form, hydrogeologic setting, and hyporheic exchange remains inadequately characterized. We used the models HEC-RAS, MODFLOW, and MODPATH to simulate coupled surface and subsurface hydraulics in a gaining stream containing a single in-stream geomorphic structure and to systematically evaluate the impact of fundamental characteristics of the structure and its hydrogeologic setting on induced exchange. We also conducted a field study to support model results. Model results indicated that structure size, background groundwater discharge rate, and sediment hydraulic conductivity are the most important factors determining the magnitude of induced hyporheic exchange, followed by geomorphic structure type, depth to bedrock, and channel slope. Model results indicated channel-spanning structures were more effective at driving hyporheic flow than were partially spanning structures, and weirs were more effective than were steps. Across most structure types, downwelling flux rate increased linearly with structure size, yet hyporheic residence time exhibited nonlinear behavior, increasing quickly with size at low structure sizes and declining thereafter. Important trends in model results were observed at the field site and also interpreted using simple hydraulic theory, thereby supporting the modeling approach and clarifying underlying processes.

[1] We examined channel response following the removal of low-head dams on two lowgradient, fine-to coarse-grained rivers in southern Wisconsin. Following removal, channels eroded large quantities of fine sediment, resulting in deposition 3-5 km downstream. At one site (Baraboo River), upstream changes were rapid and included bed degradation, minimal bank erosion, and sediment deposition on channel margins and new floodplain. Sand was transported through the former impoundment and temporarily deposited downstream. At the second site (Koshkonong River), head-cut migration governed channel adjustments. A deep, narrow channel formed downstream of the headcut, with negligible changes upstream of the head-cut. Fluvial changes were summarized in a conceptual channel evolution model that highlighted (1) similarities between adjustments associated with dam removal and other events that lower channel base-level, and (2) the role of reservoir sediment characteristics (particle size, cohesion) in controlling the rates and mechanisms of sediment movement and channel adjustment.

Despite decades of work on implementing best management practices to reduce the movement of excess nitrogen (N) to aquatic ecosystems, the amount of N in streams and rivers remains high in many watersheds. Stream restoration has become increasingly popular, yet efforts to quantify N‐removal benefits are only just beginning. Natural resource managers are asking scientists to provide advice for reducing the downstream flux of N. Here, we propose a framework for prioritizing restoration sites that involves identifying where potential N loads are large due to sizeable sources and efficient delivery to streams, and when the majority of N is exported. Small streams (1st–3rd order) with considerable loads delivered during low to moderate flows offer the greatest opportunities for N removal. We suggest approaches that increase in‐stream carbon availability, contact between the water and benthos, and connections between streams and adjacent terrestrial environments. Because of uncertainties concerning the magnitude of N reduction possible, potential approaches should be tested in various landscape contexts; until more is known, stream restoration alone is not appropriate for compensatory mitigation and should be seen as complementary to land‐based best management practices.

[1] Hydraulic fracturing has made vast quantities of natural gas from shale available, reshaping the energy landscape of the United States. Extracting shale gas, however, generates large, unavoidable volumes of wastewater, which to date lacks accurate quantification. For the Marcellus shale, by far the largest shale gas resource in the United States, we quantify gas and wastewater production using data from 2189 wells located throughout Pennsylvania. Contrary to current perceptions, Marcellus wells produce significantly less wastewater per unit gas recovered (approximately 35%) compared to conventional natural gas wells. Further, well operators classified only 32.3% of wastewater from Marcellus wells as flowback from hydraulic fracturing; most wastewater was classified as brine, generated over multiple years. Despite producing less wastewater per unit gas, developing the Marcellus shale has increased the total wastewater generated in the region by approximately 570% since 2004, overwhelming current wastewater disposal infrastructure capacity.

Dam removal is gaining credibility as a viable management option for dams that have deteriorated physically and are no longer economically practical. However, the decision to remove or repair a dam is often contentious and emotionally charged. Part of the acrimony arises from our limited scientific knowledge of the effects of dam removal. We believe that the ecological consequences are best understood by viewing the removal process as a disturbance. Ecological outcomes will include changes that are both environmentally costly, such as invasion of exotic species, and environmentally beneficial, such as increasing access to spawning habitats for migratory fish. It has also become apparent that the wholesale aging of the US dam infrastructure will make dam removal even more common in the future. The challenge ahead is to better understand and manage the consequences of these removals.

Numerous studies exist on the hydraulics of woody debris jams and the mechanisms driving their geomorphic influence. While most hydraulic studies treat jams as single, solid objects, jams are clearly not individual cylindrical logs but rather an accumulation of pieces ranging in size from leaves and twigs to entire trunks. Here we treat debris jams as complex and porous accumulations of heterogeneous material to understand the relative importance of the different size fractions comprising a jam. We systematically dismantled three deflector debris jams in four stages, removing a total of 17,783 individual wood pieces, to experimentally manipulate jam porosity. We measured the surrounding velocity, shear stress, and drag force (FD). The assumption of nonporosity can result in a 10−20% overestimation of FD. Back‐calculated values of the combined drag coefficient and frontal area term (CDAF)calc represented the drag characteristics of natural debris jams, whereas separating frontal area (AF(emp)) and drag coefficient (CD) contributions in natural jams is misleading. Values of (CDAF)calc for each jam at each stage of removal captured the effects of size and composition of the jam. Wood piece size in debris jams dictates the surface area to volume relationship. This association in turn determines the rate at which FD and (CDAF)calc change with the addition of material. Only low‐porosity jams produce the geomorphic and hydraulic characteristics commonly associated with deflector jams. Our results on natural debris jams also illustrate the importance of employing variable wood size fractions when using woody debris jams for river restoration.

The magnitude of cross-ecosystem resource subsidies is increasingly well recognized; however, less is known about the distance these subsidies travel into the recipient landscape. In streams and rivers, this distance can delimit the "biological stream width," complementary to hydro-geomorphic measures (e.g., channel banks) that have typically defined stream ecosystem boundaries. In this study we used meta-analysis to define a "stream signature" on land that relates the stream-to-land subsidy to distance. The 50% stream signature, for example, identifies the point on the landscape where subsidy resources are still at half of their maximum (in- or near-stream) level. The decay curve for these data was best fit by a negative power function in which the 50% stream signature was concentrated near stream banks (1.5 m), but a non-trivial (10%) portion of the maximum subsidy level was still found > 0.5 km from the water's edge. The meta-analysis also identified explanatory variables that affect the stream signature. This improves our understanding of ecosystem conditions that permit spatially extensive subsidy transmission, such as in highly productive, middle-order streams and rivers. Resultant multivariate models from this analysis may be useful to managers implementing buffer rules and conservation strategies for stream and riparian function, as they facilitate prediction of the extent of subsidies. Our results stress that much of the subsidy remains near the stream, but also that subsidies (and aquatic organisms) are capable of long-distance dispersal into adjacent environments, and that the effective "biological stream width" of stream and river ecosystems is often much larger than has been defined by hydro-geomorphic metrics alone. Limited data available from marine and lake sources overlap well with the stream signature data, indicating that the "signature" approach may also be applicable to subsidy spatial dynamics across other ecosystems.