Geomorphology, Hyporheic Processes, and Land-Water Interfaces
Presiding: J Marks, Northern Arizona University; D DeWalle, Penn State University
NB52E-01 10:30h
Influence of Hyporheic Exchange on Solute Transport in a Highly Hydropower-Regulated River
This study is a first step to understand the effect of hyporheic exchange on solute transport in larger rivers and to investigate whether river regulation alters the hyporheic zone interaction. The present study focuses on the effect of hyporheic exchange on inert solute transport in the Lule River, northern Sweden. A modelling framework comprising both a hydraulic and a solute transport model was developed. Recent theories for the hydrodynamics of the hyporheic exchange were used to generalize exchange relationships developed for a smaller stream system to the Lule River. The effect of hyporheic exchange on solute transport was investigated by means of numerical simulations during both regulated and unregulated conditions. The dissolved hyporheic exchange significantly affected solute transport in both the regulated and the unregulated Lule River. The hyporheic exchange caused significant retardation of an inert solute pulse. In the unregulated river, the expected residence time could be prolonged by ~100 percents compared to the case where no hyporheic exchange was present. This prolongation provides changed reaction times in the river, which can be of importance to possible chemical and biological processes. Further, hyporheic exchange caused the relative spread of an inert solute pulse to increase by up to ~11 times compared to the case where no hyporheic exchange was present. The effect was found to be largest in the unregulated river. The spreading of the pulse implies a smoothing out of temporal variations of the solute concentrations in the Lule River.
NB52E-02 10:45h
Predicting Changes in Geomorphology, Biodiversity, and Ecosystem Processes Following a Dam Removal
A large hydropower dam will be decommissioned in Fossil Creek, Arizona in 2005. Water has been diverted from the stream for over a century. A unique attribute of Fossil Creek is the high levels of calcium carbonate which historically formed large travertine dams. Travertine dams are only formed in a 1 km reach but historical records suggest that most of the river had travertine dams prior to the water diversion. Restoration of flow should increase travertine dam formation changing the geomorphology of the stream creating steep waterfalls and deep pools. We compared decomposition, nitrogen uptake length, productivity, and fish and macroinvertebrate diversity in an area of the stream with remnant travertine dams with an area with a traditional riffle/pool structure. Primary and secondary productivity, insect diversity and native fish diversity were all higher in the travertine reach. Geomorphologists expect a ten fold increase in travertine deposition following return of full flows. Dam decommissionings provide powerful experimental designs for testing how changing flow regimes affect geomorphology, biodiversity, and ecosystem processes.
NB52E-03 11:00h
Hydrogeomorphic Controls on Spatiotemporal Sediment and Nutrient Dynamics Following a Low-head Dam Removal
In this study we address sediment and nutrient export following removal of a 3 m dam on Murphy Creek, California and examine the restored reach in an effort to understand the hydrobiogeochemical mechanisms driving nutrient dynamics in the recovering system. Following removal, sediment and nutrient yields increased by an order of magnitude over the previous two years. Geomorphic surveying indicated that the majority of sediment transport occurred in pools and in the lowest 50 m of the restored reach. Phosphate export occurred primarily during large storms with the reach acting as a phosphate sink during most flow conditions. The majority of N was sourced to areas within the sediment wedge that had high extractable N concentrations (NH4+ and NO3-) and dried out on a seasonal basis. Near the restored dam site year-round water saturation inhibited nitrification and the export of N as NO3-, instead, this wetland area acted as an NH4+ sink and leached little NO3- to the channel. Nitrogen leaching from sediments occurred after removal, in autumn 2003, and again during the following autumn, indicating that N leaching from sediments is largely a seasonal process which may affect downstream aquatic ecosystems for years to come.
NB52E-04 11:15h
Hyporheic and Riparian Exchange of Water and Nitrogen in a Headwater Appalachian Forest Stream.
The traditional view of stream baseflow generation and stream chemistry control via upwelling hyporheic groundwater flow did not exist on Baldwin Creek, a 535 hectare headwater forest catchment in the Appalachians of Pennsylvania. Streambed and bank piezometers were installed at 14 points along a 4 km stream length. Head and water chemistry data were collected quarterly during 2000-2001. Stream and piezometer heads consistently showed that the stream was perched and was gradually losing water throughout its length with occasional lateral inputs through its banks. Streamflow gains occurred from numerous surface seepage zones and springs that emanated from sandstone outcrops along the channel and flood plain. Piezometer water showed high ammonium, DON, and DOC concentrations compatible with reducing conditions, while stream water was dominated by nitrate added by individual seeps and springs. Maintenance of riparian ecosystem function in such a setting is as dependent upon protection of seeps, springs, and their source areas as the main stream itself.
NB52E-05 11:30h
Watershed Conservation and Groundwater Management: An Integrated Perspective
US natural resource policy has explicitly acknowledged the hydrological connection between forest resources and water resources from the inception of the USDA Forest Service for the dual purpose of timber and watershed management,, but it is often overlooked in short run policy decisions. In Hawaii, these closely interconnected resources led to the establishment of the Ko`olau Mountains Conservation District in the early 1900s in order to improve water supplies. This early action on the part of the state has enabled today a healthy watershed. The health of the watershed, however, is now under threat from incremental ecosystem change, particularly in the form of invasive species (e.g. pigs (Sus scrofa) and weedy shrubs (Miconia calvescens)) that change the hydrological properties of the watershed to increase runoff and reduce aquifer recharge. Economic costs of reduced recharge in the face of rising water demand from a growing population are potentially large, with preliminary estimates suggesting the losses from reduced groundwater recharge in the Pearl Harbor aquifer have a present value of $1.4 to $2.6 billion dollars (Kaiser and Roumasset, 2002). To refine and improve these preliminary estimates we use spatial analysis of the water balance in the Ko`olaus to relate land use and land cover to recharge and we simultaneously explore the risk of degradation of the forest quality for recharge purposes through a survey of watershed experts. Using this information together with a dynamic model of water pricing as a function of aquifer recharge and use, we examine how much of an economic return (in present value) forest conservation expenditures may produce in the form of protecting aquifer recharge. In conjunction, we begin to examine additional integrated benefits of reducing runoff to near-shore resources by relating upland conservation to reef quality using monitoring data from the Hawaii Coral Reef Assessment and Monitoring Program. Kaiser and Roumasset (2002). "Valuation of Nature's Intermediate products: The Ko`olau Forest's Contribution to the Pearl Harbor Aquifer," Environment and Development Economics 7(4): 701-714.
NB52E-06 11:45h
Integrating Vegetation, Soil and Topography to Assess the Impact of Lateral Flow on Plant Solute Uptake
Simulation of solute uptake by vegetation in complex terrain typically fails to account for subsurface lateral movement of solutes. This study uses a spatially explicit plant-soil-water simulation model to investigate whether subsurface lateral flow at the sand-clay interface impacts tritium uptake by mixed forest vegetation. Ten hectares of a mixed pine - laurel oak forest on Coastal Plain soils periodically received irrigation with tritium-enriched water (activity ranged from 5,000 to 20,000 pCi/ml) over a three year time period. To simulate water and tritium fluxes we developed a spatially explicit water balance model. Tritium was completely mixed daily with water in each soil layer. Vertical flow of water was simulated using a capacitance model with lateral flow dependent on head development and the local slope of the impeding clay layer. The model was evaluated by comparing biweekly measurements of tritium activity (measured to 3 meter depth) and soil water content (measured to 2 meter depth) in 18 measurement clusters distributed over the catchment. We evaluated the importance of including subsurface flow in model simulations. Lateral flow was locally important (mean distance tritium traveled laterally was 1.35 m). However, after three years of simulation, the maximum predicted lateral movement of tritium did not exceed 70 meters. On the catchment scale, the average simulated amount of tritium taken up by vegetation was not impacted by lateral flow, but smaller scale spatial variability in tritium uptake increased with the inclusion of lateral flow. Simulated tritium uptake was most sensitive to changes in vegetation cover, and was less sensitive to differences in soil properties (e.g. field capacity, hydraulic conductivity and root distribution). When integrated over the study area, the simulation of solute uptake by a mixed forest in Coastal Pain soils was not sensitive to inclusion of subsurface lateral flow of water.