Steven M. Wondzell

Seasonal and storm dynamics of the hyporheic zone of a 4th-order mountain stream. I: Hydrologic processes

The objective of this study was to quantify fluxes of ground water and advected channel water through the shallow aquifer adjacent to a 4th-order mountain stream. A network of wells was installed from 1989 to 1992. Water-table elevations were measured seasonally and during storms. These data were used to calibrate MODFLOW, a 2-dimensional groundwater flow model. The fluxes of water through the subsurface were estimated from the head distributions predicted by the model for 8 steady state model runs bracketing the observed range in baseflow conditions, and for 1 transient simulation of a large storm. The overall pattern of subsurface flow changed little over the course of the year, even though the relative flux of advected channel water and ground water changed among seasons and during storms. Apparently the longitudinal gradient of the main valley, the location of the stream, and the influence of secondary channels determined the pattern of subsurface flows. Subsurface fluxes through a gravel bar were dominated by advected channel water but fluxes through the floodplain were dominated by ground water. Flow rates were positively correlated to estimated stream discharge during base-flow periods, but decreased slightly during storms because of precipitation inputs to the aquifer. The mean residence time of water stored within the aquifer was approximately 10 d for the gravel bar and 30 d for the floodplain during baseflow periods. Even though precipitation during the simulated storm equaled 12% and 23% of the water stored in the gravel bar and the floodplain, respectively, the mean residence time of water remained long.

Disturbance regimes of stream and riparian systems — a disturbance-cascade perspective

Geomorphological processes that commonly transport soil down hillslopes and sediment and woody debris through stream systems in steep, mountainous, forest landscapes can operate in sequence down gravitational flowpaths, forming a cascade of disturbance processes that alters stream and riparian ecosystems. The affected stream and riparian landscape can be viewed through time as a network containing a shifting mosaic of disturbance patches — linear zones of disturbance created by the cascading geomorphological processes. Ecological disturbances range in severity from effects of debris flows, which completely remove alluvium, riparian soil and vegetation along steep, narrow, low-order channels, to localized patches of trees toppled by floating logs along the margins of larger channels. Land-use practices can affect the cascade of geomorphological processes that function as disturbance agents by changing the frequency and spatial pattern of events and the quantity and size distribution of material moved. A characterization of the disturbance regime in a stream network has important implications for ecological analysis. The network structure of stream and riparian systems, for example, may lend resilience in response to major disturbances by providing widely distributed refuges. An understanding of disturbance regime is a foundation for designing management systems. Copyright

Seasonal and Storm Dynamics of the Hyporheic Zone of a 4th-Order Mountain Stream. II: Nitrogen Cycling

The objective of this study was to quantify subsurface nitrogen fluxes between a riparian forest and a 4th-order mountain stream, McRae Creek, for each season of the year and during storms. A network of wells was installed on a gravel bar and a portion of the adjacent floodplain between 1989 and 1992. Water samples were collected to monitor dissolved nitrogen concentrations. Advected channel water and ground water were enriched in nitrogen relative to the stream; thus, subsurface flow was a net source of nitrogen to the stream in all seasons of the year and during both base-flow periods and storms. Estimates of the flux of advected channel water and the discharge of ground water were combined with changes in mean nitrogen concentrations along subsurface flow paths to estimate nitrogen inputs to the stream. Discharge of ground water from the conifer-dominated flood-plain was the largest source of nitrogen added to the stream: however, more than 50% of this nitrogen was dissolved organic nitrogen. In contrast, two-thirds of the nitrogen from the alder-dominated gravel bar was inorganic. Net nitrogen fluxes from the gravel bar to the stream were lowest during the summer when water table elevations were low. Net fluxes of nitrogen from the gravel bar to the stream were largest during the fall, especially at peak flow during storms when interstitial water in the gravel bar was enriched in NO3-. The estimated annual flux of nitrogen from the riparian forest to McRae Creek was 1.9 g/m2 of streambed, of which 1.0 g/m2 was inorganic. Estimated net annual flux was large relative to the estimated input of nitrogen in litterfall, or the nitrogen required to support estimated rates of primary productivity.

Riparian forest disturbances by a mountain flood - the influence of floated wood.

Large floods can have major impacts on riparian forests. Here we examine the variability and spatial distribution of riparian forest responses along eight third- to fifth-order streams following a large flood (∼100 year recurrence interval) in the Cascade Mountain Range of Oregon. We categorized disturbance intensity (physical force) exerted on riparian trees during floods into three classes: (i) purely fluvial (high water flow only); (ii) fluvial supplemented by dispersed pieces of floating wood (uncongested wood transport); (iii) fluvial with movement of batches of wood (congested wood transport). These types of material transport and associated classes of disturbance intensity resulted in a gradient of biotic responses of disturbance severity ranging from standing riparian trees inundated by high water, to trees toppled but still partially rooted, to complete removal of trees. High within-stream and among-stream responses were influenced by pre-flood stream and riparian conditions as well as flood dynamics, especially the availability of individual pieces or congested batches of wood. Fluvial disturbance alone toppled fewer riparian trees than in reaches where floodwaters transported substantial amounts of wood. Debris flows delivered additional wood and sediment to parts of reaches of four of these study streams; riparian trees were removed and toppled for up to 1·5 km downstream of the debris flow tributary channel. Congested wood transport resulted in higher frequency of toppled trees and greater deposition of new wood levees along channel margins. The condition of the landscape at the time of a major flood strongly influenced responses of riparian forests. Recent and historic land-use practices, as well as the time since the previous large flood, influenced not only the structure and age of the riparian forests, but also the availability of agents of disturbance, such as large pieces of floating wood, that contribute to disturbance of riparian forests during floods. Copyright

Coupling multiscale observations to evaluate Hyporheic nitrate removal at the reach scale

Excess NO3– in streams is a growing and persistent problem for both inland and coastal ecosystems, and denitrification is the primary removal process for NO3–. Hyporheic zones can have high denitrification potentials, but their role in reach- and network-scale NO3– removal is unknown because it is difficult to estimate. We used independent and complementary multiscale measurements of denitrification and total NO3– uptake to quantify the role of hyporheic NO3– removal in a 303-m reach of a 3rd-order agricultural stream in western Oregon, USA. We characterized the reach-scale NO3– dynamics with steady-state 15N-NO3– tracer-addition experiments and solute-transport modeling, and measured the hyporheic conditions via in-situ biogeochemical and groundwater modeling. We also developed a method to link these independent multiscale measurements. Hyporheic NO3– removal (rate coefficient λHZ = 0.007/h) accounted for 17% of the observed total reach NO3– uptake and 32% of the reach denitrification estimated from the 15N experiments. The primary limitations on hyporheic denitrification at the reach scale were availability of labile dissolved organic C and the restricted size of the hyporheic zone caused by anthropogenic channelization (sediment thickness ≤1.5 m). Linking multiscale methods made estimates possible for hyporheic influence on stream NO3– dynamics. However, it also demonstrated that the traditional reach-scale tracer experimental designs and subsequent transport modeling cannot be used alone to directly investigate the role of the hyporheic zone on reach-scale water and solute dynamics.

Groundwater–surface-water interactions: current research directions

National Institute of Water and Atmospheric Research, Christchurch, New Zealand Department of Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado 80523 USA Department of Civil and Environmental Engineering, Northwestern University, Evanston, Illinois 60208 USA Odum School of Ecology, University of Georgia, Athens, Georgia 30602 USA Pacific Northwest Research Station, United States Department of Agriculture Forest Service, Corvallis, Oregon 97331 USA

Groundwater–surface-water interactions: perspectives on the development of the science over the last 20 years

Freshwater Science published a special series of papers on groundwater–surface-water (GW-SW) interactions in this issue (2015), marking the anniversary of an earlier special series of papers on the hyporheic zone published in 1993. In this concluding paper, I compare the 2 special series of papers and use this comparison to examine the development of the science in the years between 1993 and 2015. The 1993 papers marked the beginning of a period of exponential growth in the study of, and publication of, papers on GW-SW interactions. The 1993 papers tended to be forward looking, proposing conceptual models of GW-SW interactions across stream networks and identifying critical gaps. The 2015 special series of papers contrasts sharply with that of 1993. Broad issue papers are mostly lacking from the current special series. Instead, the special series is dominated by papers focusing on process-based or descriptive studies using empirical approaches. This difference probably stems from major methodological advancements over the past 2 decades that make it possible to study GW-SW interactions with ever greater detail and, thus, allows more complete understanding of specific processes. In contrast, the authors of the 1993 special series were acutely aware of the paucity of GW-SW studies and, thus, posed their conceptual models as hypotheses. Surprisingly, these hypotheses have not been rigorously tested in the decades since their publication. Perhaps it is time to re-examine such broad conceptual models. There remains a critical need for a holistic understanding of how GW-SW interactions vary among streams types and sizes and with changes in discharge among seasons or over storm events and how these GW-SW interactions influence stream ecosystem processes.

Dynamic hyporheic and riparian flow path geometry through base flow recession in two headwater mountain stream corridors

The hydrologic connectivity between streams and their valley bottoms (stream corridor) is a critical determinant of their ecological function. Ecological functions are known to be spatially and temporally variable, but spatial dimensions of the problem are not easily quantified and thus they are usually overlooked. To estimate the spatial patterns of connectivity, and how connectivity varies with changes in discharge, we developed the hyporheic potential model. We used the model to interpret a series of solute tracer injections in two headwater mountain streams with contrasting valley bottom morphologies to estimate connectivity in the stream corridor. The distributions of flow path origination locations and the lengths of hyporheic flow paths appear to vary with base flow recession, even in cases where transport timescales are apparently unchanged. The modeled distribution of origination locations further allowed us to define a spatial analog to the temporal window of detection associated with solute tracer studies, and enables assessment of connectivity dynamics between streams and their corridors. Altogether, the reduced complexity hyporheic potential model provides an easy way to anticipate the spatial distribution and origination locations of hyporheic flow paths from a basic understanding of the valley bottom characteristics and solute transport timescales. Plain Language Summary The manuscript details a simple method to assess the spatial connectivity of streams and their riparian zones. While the timescales of exchange in the river corridor have been broadly studied, the complimentary spatial dimension (i.e., the geometry of exchange flowpaths) remains largely unknown. The major challenge in assessing the spatial dimensions of exchange is the limited information available in the subsurface. Here, we develop a reduced complexity model of valley bottom transport to overcome these information limitations. With this model, relatively simple field site characterization and solute tracer data are combined to assess the spatial distribution of downwelling along a headwater mountain stream. We validate the model with a numerical experiment, and demonstrate its application in two watersheds of contrasting geology, repeated through baseflow recession.