Middle Fork Flathead Research Articles

Terrestrial and Aquatic Mollusks of the Eocene Kishenehn Formation, Middle Fork Flathead River, Montana

The Kishenehn Formation of northwest Montana and southeast British Columbia hosts one of the most prolific and diverse assemblages of fossil nonmarine mollusks in North America. More than 76 taxa, of which 41 are newly described species, of middle Eocene to early Miocene terrestrial and aquatic gastropods have been collected and described. This paper reports on the discoveries made in the oldest part of the formation that is middle Eocene in age in the southernmost part of the Kishenehn Basin. Outcrops along the Middle Fork of the Flathead River from this area have yielded over 1400 specimens from which 62 taxa (37 terrestrial, 25 aquatic) were identified. Of these, eight are new species and nine were described elsewhere but were not previously reported in the formation. In terms of paleoenvironmental affinities, the fauna are divisible into three groups. Group I is a relict tropical wet fauna, existing in basinal waterways whose modern analogs live in the Caribbean and Central and South America. Group II is a subtropical, semi-arid fauna with modern analogs ranging from the north coast of the Gulf of Mexico, across the southern United States and northern Mexico to southern California and the Baja Peninsula. Group III is a humid, warm temperate fauna that was transported from mountains east of the basin and deposited in fan-delta and lake-margin settings. Modern analogs of these molluscan taxa are common in the east-central United States. The unusual coexistence of these climatically disparate groups of fossil mollusks is a manifestation of the large paleotopographic relief, perhaps greater than 2000 m, that existed between the valley floor and the mountains. The following taxa are described herein by the senior author (HGP) as new species: Bivalvia: Sphaeriidae- Sphaerium lentum, Pisidium triangulatum. Gastropoda: Proserpinellidae- Tozerpina elsae; Subulinidae- Obeliscus scalplockensis; Planorbidae- Ferrissia circus, Rhodacmaea crepida, Omalodiscus spira; Physidae- Physella olive; and one new subspecies: Gastropoda: Pupillidae- Pupilla hebes sinistra.

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.

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.

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.

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.

POTENTIAL CLIMATE CHANGE IMPACTS ON MOUNTAIN WATERSHEDS IN THE PACIFIC NORTHWEST1

ABSTRACT: Global climate change due to the buildup of greenhouse gases in the atmosphere has serious potential impacts on water resources in the Pacific Northwest. Climate scenarios produced by general circulation models (GCMs) do not provide enough spatial specificity for studying water resources in mountain watersheds. This study uses dynamical downscaling with a regional climate model (RCM) driven by a GCM to simulate climate change scenarios. The RCM uses a subgrid parameterization of orographic precipitation and land surface cover to simulate surface climate at the spatial scale suitable for the representation of topographic effects over mountainous regions. Numerical experiments have been performed to simulate the present‐day climatology and the climate conditions corresponding to a doubling of atmospheric CO2 concentration. The RCM results indicate an average warming of about 2.5°C, and precipitation generally increases over the Pacific Northwest and decreases over California. These simulations were used to drive a distributed hydrology model of two snow dominated watersheds, the American River and Middle Fork Flathead, in the Pacific Northwest to obtain more detailed estimates of the sensitivity of water resources to climate change. Results show that as more precipitation falls as rain rather than snow in the warmer climate, there is a 60 percent reduction in snowpack and a significant shift in the seasonal pattern of streamflow in the American River. Much less drastic changes are found in the Middle Fork Flathead where snowpack is only reduced by 18 percent and the seasonal pattern of streamflow remains intact. This study shows that the impacts of climate change on water resources are highly region specific. Furthermore, under the specific climate change scenario, the impacts are largely driven by the warming trend rather than the precipitation trend, which is small.

A distributed hydrology‐vegetation model for complex terrain

A distributed hydrology‐vegetation model is described that includes canopy interception, evaporation, transpiration, and snow accumulation and melt, as well as runoff generation via the saturation excess mechanisms. Digital elevation data are used to model topographic controls on incoming solar radiation, air temperature, precipitation, and downslope water movement. Canopy evapotranspiration is represented via a two‐layer Penman‐Monteith formulation that incorporates local net solar radiation, surface meteorology, soil characteristics and moisture status, and species‐dependent leaf area index and stomatal resistance. Snow accumulation and ablation are modeled using an energy balance approach that includes the effects of local topography and vegetation cover. Saturated subsurface flow is modeled using a quasi three‐dimensional routing scheme. The model was applied at a 180‐m scale to the Middle Fork Flathead River basin in northwestern Montana. This 2900‐km2, snowmelt‐dominated watershed ranges in elevation from 900 to over 3000 m. The model was calibrated using 2 years of recorded precipitation and streamflow. The model was verified against 2 additional years of runoff and against advanced very high resolution radiometer based spatial snow cover data at the 1‐km2 scale. Simulated discharge showed acceptable agreement with observations. The simulated areal patterns of snow cover were in general agreement with the remote sensing observations, but were lagged slightly in time.

Fire regimes of western larch – lodgepole pine forests in Glacier National Park, Montana

We conducted a detailed investigation of fire frequencies, patterns of fire spread, and the effects of fire on tree succession in the western larch – lodgepole pine (Larixoccidentalis – Pinuscontorta var. latifolia) forests west of the Continental Divide in Glacier National Park, Montana. Master fire chronologies for 1650 to the present were constructed based on tree fire scars and fire-initiated age-classes. Two kinds of primeval fire regimes were identified: (i) a mixed-severity regime ranging from nonlethal underburns to stand-replacing fires at mean intervals of 25–75 years and (ii) a regime of infrequent stand-replacing fires at mean intervals of 140–340 years. The former regime is characteristic of the North Fork Flathead valley and appears to be linked to a relatively dry climate and gentler topography compared with the McDonald Creek – Apgar Mountains and Middle Fork Flathead areas, where the latter fire regime predominates. Fire frequency in the entire North Fork study area was 20 fire years per century prior to 1935 and 2 per century after 1935. In the other two study areas it was 3–5 per century both before and after 1935. We suggest that fire suppression has altered the primeval fire regime in the North Fork, but not in the central and southern areas.