ABSTRACT: In large floodplain rivers, hyporheic (subsurface) flow- paths transfer nutrients from productive riparian terraces to olig- otrophic off-channel habitats. Because dissolved organic carbon (DOC) fuels microbial processes and hyporheic microorganisms rep- resent the first stage of retention and transformation of these nutrients, understanding DOC flux can provide information on the constraints of microbial metabolism in the hyporheic zone of rivers. We monitored hydrology, physicochemical indicators, and dissolved organic carbon (DOC) dynamics during low and high discharge periods in the hyporheic zone of a riparian terrace on the Queets River, Washington, to understand what processes control the supply of carbon to subsurface microbial communities. As discharge increased, terrace hyporheic flowpaths changed from parallel to focused, and the location of surface water inputs to the terrace shifted from the terrace edge to head. Overall, DOC concentrations decreased along hyporheic flowpaths; however, concentrations at points along the flowpaths varied with position along the head gra- dient and age of the overlying vegetation. We estimated that there is insufficient DOC in advecting surface water to support hyporheic microbial metabolism in this riparian terrace. These trends indi- cate that there are additional carbon sources to the subsurface water, and we conclude that DOC is leaching from overlying ripari- an soils within the forest patches. Thus, subsurface DOC concentra- tions reflect a balance between surface water inputs, metabolic uptake, and allochthonous inputs from forest soils. (KEY TERMS: aquatic ecosystems; hyporheic zone; riparian; flood- plain river; dissolved organic carbon.) INTRODUCTION Hyporheic linkages between riparian zones and stream channels supply dissolved nutrients and car- bon that are fundamental for maintaining benthic algal and microbial communities, important compo- nents of riverine food webs. Riparian soils, with their high nutrient stocks and leaching rates (Bechtold, 2000), are important sources of nutrients and carbon supporting river productivity. The importance of stream processes dependent upon subsurface linkages argues for the inclusion of such processes in the design of riparian and stream restoration projects and conservation strategies. The hyporheic zone is the saturated subsurface sediment directly beneath and lateral to the wetted channel that contains some proportion of surface water (Triska et al., 1989; Edwards, 1998; Brunke and Gosner, 1997). Hyporheic flows occur at various spa- tial scales (Dahm et al., 1998; Edwards, 1998) and dif- fer from ground water flows in the reciprocal nature of exchange. Short hyporheic flowpaths (one meter to tens of meters) occur within active channel sediments and underneath riparian vegetation at the scale of pools, riffles, and individual gravel bars. Localized areas of high algal production develop where these flowpaths deliver nutrient enriched hyporheic water to the surface stream (Valett et al., 1994; Fevold, 1998). In large floodplain rivers, regions of upwelling and downwelling related to constrained and uncon- strained valley floors result in long hyporheic flow- paths of thousands of meters (Stanford and Ward, 1993). At these larger scales, preferential flowpaths formed by previous channel locations (paleochannels) are conduits of water beneath riparian zones devel- oped on river-deposited sediments (Stanford and Ward, 1993). In Pacific Northwest (PNW) floodplain rivers, floods, sediment, and large woody debris interact to maintain a mosaic of vegetation patches of differing age classes and species on fluvial land forms created by channel migration (Fetherston et al., 1995; Naiman 1 Paper No. 01044 of the Journal of the American Water Resources Association. Discussions are open until February 1, 2003. 2 Respectively, Center for Streamside Studies, College of Forest Resources, Box 352100, University of Washington, Seattle, Washington 98195; Research Ecologist, Aquatic/Land Interactions Research Program, Pacific Northwest Research Station, USDA Forest Service, 2770 Sherwood Lane, Suite 2A, Juneau, Alaska 99801; and Professor, School of Aquatic and Fishery Sciences, Box 355020, University of Washing- ton, Seattle, Washington 98195 (E-Mail/Clinton: sclinton@u.washington.edu). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 619 JAWRA JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION VOL. 38, NO. 3 AMERICAN WATER RESOURCES ASSOCIATION JUNE 2002 FOREST-RIVER INTERACTIONS: INFLUENCE ON HYPORHEIC DISSOLVED ORGANIC CARBON CONCENTRATIONS IN A FLOODPLAIN TERRACE 1 Sandra M. Clinton, Rick T. Edwards, and Robert J. Naiman 2