Nyack Floodplain Research Articles

Amphibitic stoneflies (Plecoptera) are integrators of ecosystem processes in alluvial aquifers of gravel‐bed river floodplains

AbstractOver 50 years ago nymphs of the Plecoptera species, Paraperla frontalis Banks, 1906 (Plecoptera: Chloroperlidae), were shown to exist in a shallow floodplain aquifer of the Tobacco River, a gravel‐bed river in western Montana and later they were documented throughout the main stems of the Flathead River system. Nymphs are almost never found in surface waters, until they emerge on the river shorelines. As teneral adults, they mate and subsequently deposit fertilized eggs into the river. This novel life cycle is termed “amphibitic.” Over the years we and others have found P. wilsoni Ricker, 1965 (Plecoptera: Chloroperlidae), Kathroperla perdita Banks, 1920 (Plecoptera: Kathroperlidae), and five species of Isocapnia Banks, 1938 (Plecoptera: Capniidae), including long‐winged and brachypterous adults and wingless dwarfs (male and female), occupying amphibitic niches in the alluvial aquifers of rivers in Washington, Idaho, Colorado, Alaska and British Columbia. These stoneflies are remarkably tolerant of hypoxia which allows them to exist as abundant consumers in aquifer food webs subsidized by ancient methane. Indeed, stonefly tissues contain carbon that is up to 7000 years old, underscoring the existence of a strong interaction involving the uptake of labile carbon derived from methanogenic and methanotrophic process in aquifers. Details of life cycles, trophic relationships, distribution and abundance have been documented by a suite of studies on the Nyack Floodplain of the Middle Flathead River, Montana. In this paper we review the ecophysiology and ecology of these unique stoneflies in the context of their functional role in gravel‐bed river ecosystems.This article is categorized under: Water and Life > Nature of Freshwater Ecosystems Water and Life > Conservation, Management, and Awareness

Resilient aquifer stoneflies handle low oxygen well

Sweeping through Montana's mighty Rocky Mountains, the majestic Flathead River courses through pristine valleys and canyons. But according to Rachel Malison from The University of Montana, USA, the river extends well beyond the babbling torrent visible at the surface, flooding deep into the gravel- and cobblestone-packed aquifer beneath. And these subterranean waterways are bursting with life. In the 1970s, Jack Stanford from the University of Montana, USA, found that stonefly nymphs – some of the most vulnerable inhabitants of rivers as they require well-oxygenated water – are remarkably content to reside in aquifers, even though the oxygen levels can plunge as low as 0.14 mg l–1. In addition, Amanda DelVecchia had recently discovered that the stonefly occupants of the Flathead River's Nyack aquifer dine on microorganisms that only thrive in deoxygenated environments, deepening the paradox. How could insects that usually perish when oxygen levels dwindle thrive in an aquifer where the oxygen supply can be patchy in places?After travelling through the dramatic landscape to the Nyack floodplain, Malison and intern Hailey Jacobsen pumped water from the wells drilled by Stanford in the 1980s to collect the subterranean stoneflies. ‘Sometimes we didn't get any, sometimes we might get 100’, says Malison, adding that additional creatures, including tiny crustaceans and other microscopic invertebrates, also appeared in the water. Back at the Flathead Lake Biological station, Malison identified three stonefly species (Paraperla frontalis, Isocapnia spp. and Kathroperla perdita) before measuring the insects’ oxygen consumption to calculate their metabolic rate as she steadily reduced their oxygen supply from 12 to 0.5 mg dissolved oxygen l–1.Comparing the metabolic rates of the aquifer stoneflies with stoneflies that had been collected from the river, Malison found that the aquifer residents had a much lower initial metabolic rate than the surface-dwelling insects. In addition, the species in the aquifer coped much better as the oxygen levels fell – their metabolic rate only dropping ∼46% when the oxygen was reduced by 83% – in contrast to the river dwellers’ metabolic rates, which dropped by 66%. And when Malison and Jacobsen checked the insects’ recovery after their oxygen ran out, the species that live in streams never regained their full metabolic rate, while the aquifer residents bounced back to almost normal.Knowing that the aquifer-based species could nip in and out of oxygen-depleted water if they are to dine on the specialised microorganisms that thrive there, Malison and Jacobsen then tested how P. frontalis and Isocapnia fared when the oxygen levels dropped from 12 to 0.5 mg l–1 three times over a 9 h period. Impressively, all of the stoneflies survived oxygen levels that would probably have proved fatal for the river-dwelling species; however, the reduction in P. frontalis’ metabolic rate suggests that visiting deoxygenated regions briefly to dine may be stressful. And when Malison, Julia Cotter and Haley Dole embarked on the Herculean task of continually recording the insects’ metabolic rates over a week when the animals repeatedly went without oxygen overnight after the levels had declined gradually during the day, K. Perdita was most robust (91% survival), although Isocapnia seemed to struggle more (38%).Even though most stonefly species are notoriously sensitive to the oxygen levels in their water, the stoneflies that have descended beneath the ground and made their homes in aquifers seem to be more resilient, tolerating oxygen levels that would be fatal for river dwellers. ‘This tolerance suggests that they can enter zones of low oxygen to forage and exploit hot spots in the underground aquifer where food is abundant’, says Malison.

Methane-derived carbon supports a complex food web in the shallow aquifer

Although most life on earth is fueled by photoautotrophic food resources, there is increasing evidence for methanogenic methane-derived carbon contributions to freshwater consumers. In one extreme case on the Nyack floodplain in Montana, methane-derived carbon comprised a majority of biomass carbon in stoneflies (Order: Plecoptera) living in shallow aquifers underlying river floodplains, helping to explain how large organisms survive in such a carbon-limited system. We studied the food web structure of this system, including five stonefly species, two meiofauna taxa, and one amphipod. We found that trophic positions that varied within and between species were distinguished by δ13C and δ15N signatures indicative of varying, and sometimes preferential, use of methane-derived carbon resources, as well as some combination of trophic level variation within resource classifications. Some of these species likely access food resources at oxic-anoxic interfaces, as suggested by the presence of methanogenic and methanotrophic sequences in gut contents, as well as some evidence for anoxia tolerance. Overall, our work suggests that organisms in shallow aquifer ecosystems have adapted to coexist and use unconventional carbon sources that form in biogeochemically heterogeneous conditions.

Dissolved organic carbon lability increases with water residence time in the alluvial aquifer of a river floodplain ecosystem

AbstractWe assessed spatial and temporal patterns of dissolved organic carbon (DOC) lability and composition throughout the alluvial aquifer of the 16 km2 Nyack Floodplain in northwest Montana, USA. Water influx to the aquifer derives almost exclusively from the Middle Fork of the Flathead River, and water residence times within the aquifer range from days to months. Across seasons and channel discharge conditions, we measured DOC concentration, lability, and optical properties of aquifer water sampled from 12 wells, both near and ~3 m below the water table. Concentrations of DOC were typically low (542 ± 22.7 µg L−1; mean ± se), and the percentage of labile DOC averaged 18 ± 12% during 3 day laboratory assays. Parallel factor analysis of fluorescence excitation‐emission matrices revealed two humic‐like and two amino acid‐like fluorescence groups. Total DOC, humic‐like components, and specific UV absorbance decreased with water residence time, consistent with sorption to aquifer sediments. However, labile DOC (both concentration and fraction) increased with water residence time, suggesting a concurrent influx or production of labile DOC. Thus, although the carbon‐poor, oxygen‐rich aquifer is a net sink for DOC, recalcitrant DOC appears to be replaced with more labile DOC along aquifer flow paths. Our observation of DOC production in long flow paths contrasts with studies of hyporheic DOC consumption along short (centimeters to meters) flow paths and highlights the importance of understanding the role of labile organic matter production and/or influx in alluvial aquifer carbon cycling.

Combining active and passive hydroacoustic techniques during flood events for rapid spatial mapping of bedload transport patterns in gravel-bed rivers

With 7 figures Abstract: Turbulent flow in rivers and the associated movement of sediment creates unique underwater sound- scapes that can be measured passively with hydrophones while Acoustic Doppler Profilers (ADP) are an active form of hydroacoustic sampling that can be used to provide a surrogate measurement of Apparent Bedload Veloc- ity (ABV). In our study, longitudinal profiles of ADP and sound were simultaneously measured, while floating the river in a raft on the Nyack Floodplain of the Middle Fork of the Flathead River, USA during flood events exceeding bankfull conditions. In addition, similar measurements were carried out on the Kootenai River during a prescribed flood release aimed at mobilizing gravel bed sediment to positively impact White Sturgeon spawn- ing. Both data sets revealed spatially explicit zones of coherent ABV and bedload intensity (sound) over the two 12 km river segments. The ability to remotely and in real-time assess bedload transport for large gravel-bed rivers, on the floodplain scale, is a missing piece of information important for basic ecological understanding and applied science, specifically management decisions regarding regulated rivers worldwide. With these data sets we dem- onstrate a new methodology for rapid real-time spatial surveying of bedload transport in large gravel-bed rivers.

Landscape Influences on Ecosystem Function: Local and Routing Control of Oxygen Dynamics in a Floodplain Aquifer

Perspectives addressing either “local” or “routing” control over ecological form and function alternatively emphasize ambient conditions or recognize that form and processes at one location may be heavily influenced by other points in space. In their natural state, floodplains of montane gravel-bed rivers are organized laterally into parafluvial zones, where scour occurs annually, and orthofluvial environments, within which active (that is, early successional) and passive (that is, late successional) zones differ in regard to flooding influences. Vertically, these rivers interact with extensive alluvial aquifers throughout the length of their floodplains. We addressed how local and routing controls organize riverscape structure and function by investigating seasonal dynamics of interstitial dissolved oxygen (DO) vertically within the alluvial aquifer, longitudinally along flowpaths, and laterally across active and passive orthofluvial (PO) zones in the Nyack floodplain, Middle Fork Flathead River, MT. Groundwater DO was positively correlated to pH and negatively correlated electrical conductivity (EC) across all seasons. In some wells (7 of 19), DO was significantly lower and EC greater at shallow depths, and the magnitudes of vertical zonation were related to dissolved organic carbon concentrations. Longitudinal declines in DO were always and exclusively observed along flowpaths of the PO zone. At the same time, persistent hypoxic patches within the active orthofluvial environment suggest metabolic hotspots that contribute to local control of physicochemistry. In ecosystems heavily influenced by material transport, local and routing controls may be manifested simultaneously as distinct patterns of organization, which are expressed at different spatial and temporal scales.

Spatial drivers of ecosystem structure and function in a floodplain riverscape: springbrook nutrient dynamics

On riverine flood plains, reorganization by fluvial processes creates and maintains a mosaic of aquatic and riparian landscape elements across a biophysical gradient of disturbance and succession. Across flood plains of gravel-bottom rivers, spring brooks emerge from points of groundwater discharge that may occur in distinct landscape positions. We investigated how ecosystem processes in spring brooks differ spatially across biophysical zones, reflecting how landscape position dictates severity of flood disturbance, allochthonous loading from riparian forests, and inputs from groundwater systems. Between July and October 2011, we quantified aspects of ecosystem structure and function among 6 spring brooks of the Nyack flood plain, Flathead River, Montana. Structural features varied predictably across near-channel (i.e., parafluvial) and late successional (i.e., orthofluvial) biophysical zones. Large wood standing stocks increased >40× (0.19–9.19 kg/m2), dominant particle size class differed by an order of magnitude (median particle size [D50] = 2–27), and measures of vertical hydraulic gradient (–0.06 to +0.12 cm/cm) reflected differences in landscape position. We found fine sediment accumulation, stronger groundwater inputs, and greater benthic and large wood standing stocks in orthofluvial than in parafluvial spring brooks. Algal biomass was negatively correlated with insolation and positively related to vertical hydraulic gradient. Results from microcosm experiments showed increasing N uptake across the gradient from parafluvial to orthofluvial spring brooks. Functional response to landscape-scale organization of springbrook structure underscores the need for a spatially explicit model of floodplain ecology.

Dissolved oxygen and dissolved inorganic carbon stable isotope composition and concentration fluxes across several shallow floodplain aquifers and in a diffusion experiment

Recent studies have documented the occurrence of dissolved molecular oxygen (DO) in shallow groundwater that is isotopically lighter than can be explained by atmospheric gas exchange or by biogeochemical reactions that consume 16O16O faster than 16O18O. In the present study, spatial gradients in the isotopic composition of DO (δ18O-DO) and dissolved inorganic carbon (δ13C-DIC) were measured in three shallow floodplain aquifers: (1) the Nyack aquifer, of the Middle Fork of the Flathead River in northwest Montana; (2) the Silver Bow Creek floodplain in southwest Montana; and (3) the Umatilla River floodplain in northeast Oregon. The field data show general trends of increasing DIC concentration, decreasing δ13C-DIC, and decreasing DO concentration with increase in groundwater path length. These trends are consistent with consumption of DO and production of DIC by microbial respiration. Although the expected trend of an increase in δ18O-DO with increase in path length was found at an area adjacent to hyporheic recharge at the Nyack floodplain, the majority of groundwater samples collected at Nyack and from the other sites distal to recharge zones had anomalously low δ18O-DO values well below 24.2 ‰, the value corresponding to atmospheric isotopic equilibrium. At the Nyack site, 3H-3He dates were used to estimate groundwater travel time: all groundwater samples with apparent age >1 year had δ18O-DO<24.2 ‰. Previously it has been suggested that diffusion of O2 could be a viable mechanism to explain the existence of isotopically light DO in shallow groundwater. To test this hypothesis, laboratory experiments were conducted to measure the isotopic fractionation of O2 as it diffuses from air across a simulated capillary fringe (made from a floating layer of foam beads) into a stirred, initially anoxic, water column. As expected, 16O16O diffused faster than 16O18O, and the magnitude of isotope fractionation associated with diffusion increased with a decrease in temperature. Fractionation factors (α) calculated from these diffusion experiments were 1.0030 at 15–19 °C and 1.0048 at 8 °C. The combined field and laboratory data suggest that diffusion is an important mechanism to maintain aerobic conditions in shallow groundwater systems, allowing microbial respiration to continue at long distances (km scale) from the source of groundwater recharge.

Scaling flow path processes to fluvial landscapes: An integrated field and model assessment of temperature and dissolved oxygen dynamics in a river‐floodplain‐aquifer system

Biogeochemical cycling within river ecosystems is strongly influenced by geomorphic and hydrologic dynamics. To scale point observations of temperature and dissolved oxygen (DO) to a hydrologically complex and dynamic three‐dimensional river‐floodplain‐aquifer system, we integrated empirical models of temperature and biotic oxygen utilization with a recently published hydrogeomorphic model. The hydrogeomorphic model simulates channel flow, floodplain inundation, and surface‐subsurface water exchange on the 16 km2 Nyack Floodplain, Middle Fork Flathead River, Montana, USA. Model results were compared to observed data sets of DO to test the hypothesis that complexity in spatiotemporal patterns of biogeochemistry emerges from a comparatively simple representation of biogeochemical processes operating within a multidimensional hydrologic system. The model explained 58% of the variance in 820 DO measurements that spanned the study site longitudinally, laterally, vertically, and across river discharge conditions and seasons. We also used model results to illustrate spatial and temporal trends of temperature and DO dynamics within the shallow alluvial aquifer, which is an extensive hyporheic zone because subsurface alluvial flow paths are recharged primarily by channel water. Our results underscore the importance of geomorphic, hydrologic, and temperature dynamics in driving river ecosystem processes, and they demonstrate how a realistic representation of a river's physical template, combined with simple biogeochemical models, can explain complex patterns of solute availability.

Relative influences of the river channel, floodplain surface, and alluvial aquifer on simulated hydrologic residence time in a montane river floodplain

Current methods for characterizing the influence of geomorphic structure on river processes are not well suited for study of large rivers with extensive hydrologic exchanges between the channel, floodplain surface, and alluvial aquifer. Here we applied a spatially explicit, three-dimensional hydrologic model to simulate surface and subsurface flow within the 16-km2 Nyack Floodplain on the Middle Fork Flathead River in northwest Montana, USA. We ran the model for four years and simulated nine conservative particle releases across a range of river discharges. Our objective was to evaluate the interactions of hydrologic dynamics and floodplain structure in determining the residence time of water in the system. Particle tracking simulations revealed when river discharge was below bankfull, mean residence time of surface water decreased with river discharge. During overbank flows, however, mean residence time increased with discharge because of surface water storage on the floodplain. When surface and subsurface water were considered, mean hydrologic residence time was more than an order of magnitude greater than the residence time of surface water alone because of storage in the alluvial aquifer. Residence time also decreased exponentially with river discharge, because a smaller percentage of surface water entered the subsurface at high discharges. Our analyses illustrate the importance of considering in-channel flow, floodwater storage, and surface-subsurface exchange in determining the overall residence time of water within a river segment, especially in floodplain environments.

Tracing dissolved O 2 and dissolved inorganic carbon stable isotope dynamics in the Nyack aquifer: Middle Fork Flathead River, Montana, USA

The geochemistry and microbiology of shallow groundwater aquifers is greatly influenced by the concentration of dissolved oxygen gas (DO); however, the mechanisms that consume DO in groundwater (e.g., biotic or abiotic) are often ambiguous. The use of stable isotopes of molecular O 2 (δ 18O-DO), in conjunction with stable isotopes of dissolved inorganic carbon (δ 13C-DIC), has potential to discriminate between the various mechanisms causing DO depletion in subsurface waters. Here we report the results of spatial and seasonal changes in δ 18O-DO and δ 13C-DIC at the Nyack floodplain aquifer along the Middle Fork of the Flathead River near West Glacier, Montana, USA. Over a short, well constrained flow path (∼100 m) near a main recharge zone of the floodplain, the δ 18O-DO consistently increased as DO concentrations decreased with distance from the recharge source. Concurrently, DIC concentrations increased and δ 13C-DIC values decreased. These observations are explained by community respiration coupled with dissolution of calcite from cobbles in the aquifer matrix. When these results are compared to data from wells distributed over the entire floodplain (several km) a much less predictable relationship was observed between DO concentration and δ 18O-DO. Many wells with low DO concentrations (e.g., <125 μmol L −1 or 4 mg L −1) had anomalously low δ 18O-DO values (e.g., <20‰). Mass balance calculations show that approximately equal amounts of O 2 may be contributed to the aquifer by diffusion from the vadose zone and by advection from the river recharge. Calculations presented here suggest that diffusion across a narrow air–water interface can contribute isotopically light δ 18O-DO to the saturated zone. Possible contributions of light δ 18O-DO from other processes, such as isotopic exchange and radial oxygen loss from plant roots in or near the water table, are compared and evaluated.

Habitat Heterogeneity and Associated Microbial Community Structure in a Small-Scale Floodplain Hyporheic Flow Path

The Nyack floodplain is located on the Middle Fork of the Flathead River, an unregulated, pristine, fifth-order stream in Montana, USA, bordering Glacier National Park. The hyporheic zone is a nutritionally heterogeneous floodplain component harboring a diverse array of microbial assemblages essential in fluvial biogeochemical cycling, riverine ecosystem productivity, and trophic interactions. Despite these functions, microbial community structure in pristine hyporheic systems is not well characterized. The current study was designed to assess whether physical habitat heterogeneity within the hyporheic zone of the Nyack floodplain was sufficient to drive bacterial beta diversity between three different hyporheic flow path locations. Habitat heterogeneity was assessed by measuring soluble reactive phosphorous, nitrate, dissolved organic carbon, dissolved oxygen, and soluble total nitrogen levels seasonally at surface water infiltration, advection, and exfiltration zones. Significant spatial differences were detected in dissolved oxygen and nitrate levels, and seasonal differences were detected in dissolved oxygen, nitrate, and dissolved organic carbon levels. Denaturing gradient gel electrophoresis (DGGE) and cell counts indicated that bacterial diversity increased with abundance, and DGGE fingerprints covaried with nitrate levels where water infiltrated the hyporheic zone. The ribosomal gene phylogeny revealed that hyporheic habitat heterogeneity was sufficient to drive beta diversity between bacterial assemblages. Phylogenetic (P) tests detected sequence disparity between the flow path locations. Small distinct lineages of Firmicutes, Actinomycetes, Planctomycetes, and Acidobacteria defined the infiltration zone and alpha- and beta-proteobacterial lineages delineated the exfiltration and advection zone communities. These data suggest that spatial habitat heterogeneity drives hyporheic microbial community development and that attempts to understand functional differences between bacteria inhabiting nutritionally heterogeneous hyporheic environments might begin by focusing on the biology of these taxa.

Benthic algal response to hyporheic-surface water exchange in an alluvial river

We studied the response of benthic algae to points of hyporheic-surface water exchange in the main channel of the Middle Fork Flathead River within the Nyack Flood Plain, Montana. We examined hyporheic exchange at 120 sites using piezometers and measuring vertical hydraulic gradient (VHG), hydraulic conductivity, and vertical discharge. We removed benthic algae from a single cobble at each site, and we used VHG to group sampling sites for statistical analysis. Algal cell density and chlorophyll a concentration were significantly higher at sites with hyporheic discharge (+VHG, upwelling) compared to both sites with hyporheic recharge (−VHG, downwelling) and sites with no hyporheic-surface water exchange (=VHG, neutral) (ANOVA, P < 0.05). The assemblages of algae at upwelling sites were also significantly different from downwelling and neutral exchange sites (ANOSIM, P < 0.05). Filamentous green algae Stigeoclonium sp. and Zygnema sp. and a chrysophyte, Hydrurus foetidus (Villars) Trevisan were abundant at upwelling sites, whereas an assemblage of diatoms Achnanthidium minutissimum (Kutzing) Czarnecki, Cymbella excisa Kutzing, Diatoma moniliformis Kutzing, and Gomphonema olivaceoides Hustedt, were the most abundant taxa at downwelling and neutral exchange sites, occurring attached to, or in close association with the stalks of Didymosphenia geminata (Lyngbye) Schmidt. These data show that benthic algal communities are structured differently depending on the direction of hyporheic flux in the main channel of a large alluvial river, suggesting that hyporheic-surface exchange may influence the spatial distribution of main-channel benthic algae in rivers with hyporheic-surface water connectivity.

Inhibition of colonization by a native arbuscular mycorrhizal fungal community via Populus trichocarpa litter, litter extract, and soluble phenolic compounds

Controls on the colonization and abundance of arbuscular mycorrhizal fungi (AMF) in ecosystems are little understood and may be related to host factors, the fungal community, and soil physio-chemical properties; and changes in these variables during soil development may affect succession between mycorrhizal groups. Here we investigated the effects of litter, litter leachates, and common soluble phenolic compounds on AMF colonization of roots. In previous studies, we observed a negative correlation between increases in black cottonwood ( Populus trichocarpa) litter and AMF abundance and inoculum potential along a riparian chronosequence in northwest Montana. From this, we hypothesized that litter inputs negatively affect the native AMF community and may contribute to the shift between AMF and ectomycorrhizas. We tested the effects of cottonwood foliage and litter extract additions on the colonization of AMF of both cottonwood and Sudan grass ( Sorghum sudanese) seedlings. Addition of 5% (v/v) dried cottonwood leaves completely inhibited AMF colonization of S. sudanese. AMF colonization of S. sudanese was significantly reduced by litter extract of P. trichocarpa foliage, and colonization was negatively correlated with litter extract concentrations. Additions of aqueous litter extract significantly reduced AMF colonization of cottonwood seedlings as well. The effect of the litter extract on AMF colonization of S. sudanese did not appear to be mediated by changes in soil pH or plant biomass. Available phosphorus was higher in soil receiving highest concentration of litter extract, but not at a level expected to be inhibitory to AMF colonization. Litter additions significantly increased total soil phenolics, but with a range similar to natural soils of the Nyack floodplain. We tested pure soluble phenolic compounds common to Populus for their effect on AMF colonization by native fungi from the Nyack floodplain. All tested compounds significantly reduced AMF colonization but did not affect colonization by non-AMF root-colonizing fungi. This suggests secondary compounds present in cottonwood litter can affect colonization ability of a native AMF community. The potential mechanisms of inhibition and the relevance of these findings to AMF succession within both a single host and soil are discussed.

CLIMATE, HYDROLOGIC DISTURBANCE, AND SUCCESSION: DRIVERS OF FLOODPLAIN PATTERN

Floodplains are among the world's most threatened ecosystems due to the pervasiveness of dams, levee systems, and other modifications to rivers. Few unaltered floodplains remain where we may examine their dynamics over decadal time scales. Our study provides a detailed examination of landscape change over a 60-year period (1945-2004) on the Nyack floodplain of the Middle Fork of the Flathead River, a free-flowing, gravel-bed river in northwest Montana, USA. We used historical aerial photographs and airborne and satellite imagery to delineate habitats (i.e., mature forest, regenerative forest, water, cobble) within the floodplain. We related changes in the distribution and size of these habitats to hydrologic disturbance and regional climate. Results show a relationship between changes in floodplain habitats and annual flood magnitude, as well as between hydrology and the cooling and warming phases of the Pacific Decadal Oscillation (PDO). Large magnitude floods and greater frequency of moderate floods were associated with the cooling phases of the PDO, resulting in a floodplain environment dominated by extensive restructuring and regeneration of floodplain habitats. Conversely, warming phases of the PDO corresponded with decreases in magnitude, duration, and frequency of critical flows, creating a floodplain environment dominated by late successional vegetation and low levels of physical restructuring. Over the 60-year time series, habitat change was widespread throughout the floodplain, though the relative abundances of the habitats did not change greatly. We conclude that the long- and short-term interactions of climate, floods, and plant succession produce a shifting habitat mosaic that is a fundamental attribute of natural floodplain ecosystems.

Multiscale geomorphic drivers of groundwater flow paths: subsurface hydrologic dynamics and hyporheic habitat diversity

Application of a hydrogeologic computer model underscored the importance of geomorphic controls on groundwater and surface-water flow dynamics in the Nyack Floodplain, a montane alluvial floodplain in Montana, USA. The model represented the floodplain as a hierarchy of geomorphic patches, which facilitated analysis of model results using independent (predictor) variables at multiple scales. The analyses revealed that geomorphic structures at various spatial scales interact with the flow regime to influence the direction, magnitude, and stability of hyporheic flow within individual floodplain patches. Specifically: 1) the hydrologic flow network within the hyporheic zone is more responsive to seasonal changes in river discharge if floodplain topography is complex and aquifer properties are heterogeneous, 2) simplification of internal patch structure across the floodplain eliminates the influence of fine-scale geomorphic structures on the stability of groundwater flow paths, although the influence of patch context remains, and 3) incremental changes in river discharge can abruptly and substantially restructure the relationship between river discharge and groundwater flow patterns when events such as inundation of previously dry flood channels occur on the floodplain. We believe that ecological theories of biodiversity can be used to understand interactions among geomorphic variation, hydrologic dynamics, and the maintenance of biodiversity in the hyporheic zone if abrupt reorganization and other variations in groundwater flow paths act as disturbances to hyporheic communities. From this perspective, we used model results to develop 4 hypotheses describing the potential for causal linkages among floodplain geomorphology, hyporheic flow-path variation, hyporheic habitat diversity/stability, and hyporheic community diversity.

A patch hierarchy approach to modeling surface and subsurface hydrology in complex flood‐plain environments

AbstractGeomorphology interacts with surface‐ and ground‐water hydrology across multiple spatial scales. Nonetheless, hydrologic and hydrogeologic models are most commonly implemented at a single spatial scale. Using an existing hydrogeologic computer model, we implemented a simple hierarchical approach to modeling surface‐ and ground‐water hydrology in a complex geomorphic setting. We parameterized the model to simulate ground‐ and surface‐water flow patterns through a hierarchical, three‐dimensional, quantitative representation of an anabranched montane alluvial flood plain (the Nyack Flood Plain, Middle Fork Flathead River, Montana, USA). Comparison of model results to field data showed that the model provided reasonable representations of spatial patterns of aquifer recharge and discharge, temporal patterns of flood‐water storage on the flood plain, and rates of ground‐water movement from the main river channel into a large lateral spring channel on the flood plain, and water table elevation in the alluvial aquifer. These results suggest that a hierarchical approach to modeling ground‐ and surface‐water hydrology can reproduce realistic patterns of surface‐ and ground‐water flux on alluvial flood plains, and therefore should provide an excellent ‘quantitative laboratory’ for studying complex interactions between geomorphology and hydrology at and across multiple spatial scales. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Three-dimensional mapping of geomorphic controls on flood-plain hydrology and connectivity from aerial photos

The Nyack flood plain of the Middle Fork Flathead River, MT, USA is a 9-km anastomosed alluvial montane flood plain. Upstream from the flood plain, the river is unregulated and the catchment virtually pristine. A patchy mosaic of vegetation and channels exists on the flood-plain surface. The surface and subsurface geomorphic structures of the flood plain facilitate high hydrologic connectivity (water flux between the channel and flood plain) marked by complex seasonal patterns of flood-plain inundation, extensive penetration of channel water laterally into the alluvial aquifer, and springbrooks formed by ground water erupting onto the flood-plain surface. After delineating and classifying flood-plain “elements” (vegetation patches and channel reaches) on the flood plain, we analyzed field-based elevation survey data to identify expected relationships among flood-plain element type, surface scour frequency, and flood-plain elevation. Data analyses show that scour frequency was inversely proportional to the elevation of the flood plain above river stage, except when localized geomorphic controls such as natural levees prevent normal high flows from inundating and scouring relatively low flood-plain elements. Further, while different flood-plain element types occupy distinct elevation zones on the flood plain, the elevation of each zone above the river channel varies with localized channel entrenchment. We found that topographic variation among flood-plain elements is greater than the variation within elements, suggesting that coarse-scale flood-plain topography can be characterized by delineating flood-plain elements. Field data document strong associations between specific classes of flood-plain elements and preferential ground-water flow paths in the upper alluvial aquifer. Combined with preexisting ground penetrating RADAR (GPR) surveys, these data intimate a sinuous lattice of preferential ground-water flow paths (buried abandoned streambeds) in the upper alluvial aquifer at approximately the same elevation as the main channel's streambed. Using aerial photo interpretation and the identified relationships among element-types, elevation, and preferential ground-water flow paths, we developed a quantitative, three-dimensional characterization of surface and subsurface geomorphology across the entire flood plain to support a heuristic modeling effort investigating the influence of flood-plain geomorphology on spatio-temporal patterns of surface and ground-water flow and exchange under dynamic hydrologic regimes.