Ritzenthaler, Elizabeth. 1998. Biogeochemistry and hydrology of a forested floodplain backchannel: Riparian and hyporheic interactions. M.S.
Complex interactions among riparian forests, subsurface flowpaths, and riverine dynamics provide the focus for this thesis. Research described herein was designed to evaluate how riparian red alder (Alnus rubra) stands influence aquatic nutrient dynamics along a floodplain backchannel on the Queets River, WA. This study also explored the capacity of the backchannel to process nitrogen and phosphorus. Solute additions were conducted to quantify fluxes and label hyporheic waters. Delivery and modification of riparian-derived nutrients were investigated at two distinct scales. Reach-scale hydrology and nutrient processing patterns were investigated along 300 to 520 m of the backchannel (chapter 1). Fine-scale flowpath analysis and physico-chemical patterns were studied within a dense piezometer network (chapter 2). Chapter 1: The backchannel was always a net gaining reach during low-flow summer conditions. As waters drained adjacent terraces, nutrient laden subsurface waters traveled into the hyproheic zone and mixed with advected surface waters. Terrace fluxes delivered 4.4 * 10-3 to 1.9 * 10-2 mg/s/m NO3-N, 3.8 * 10-4 to 3.4 * 10-3 mg/s/m NH4-N, and 3.4 * 10-4 to 2.7 * 10-3 mg/s/m SRP under ambient conditions. Biotic and abiotic processing, before and after entrance into the surface water, lead to removal of 6 to 49% NO3-N, 25 to 81% NH4-N, and up to 26% of SRP inputs. The capacity to remove nutrients varied along the reach, and despite the high assimilation capacity, there was a net import of nitrogen and phosphorus to the channel. An ammonium co-injection confirmed the high nitrification potential of the backchannel and was the only time more NO3-N was released to downstream communities than had entered. Chapter 2: Subsurface flowpath structure provides a template for physico-chemical trends. After five solute additions, at least five distinct water masses were identified. Waters draining the 2 riparian terraces carried unique signals. Three other water masses were connected to surface-derived flows to varying degrees. Percent surface water content in piezometers and nominal travel times were used to classify piezometers. Dissolved oxygen, temperature, electrical conductivity, hydraulic conductivity, NO3-N, NH4-N, TDIN, and SRP were compared among response classes. Statistical flowpath analysis revealed increasing NO3-N and TDIN and decreasing dissolved oxygen, temperature, and electrical conductivity along surface-water dominated hyporheic flowpaths. High levels of NO3-N were also measured in well oxygenated terrace waters. Highest SRP concentrations were associated with low surface water content pizometers. Complex hydrological pathways support highly heterogeneous biogeochemical patterns. Subsurface riparian inputs, which drain senescing red alder terraces, clearly supply both nitrogen and phosphorus to the backchannel. Subsurface nutrient transfers, including nitrification of surface-derived waters, are the main nutrient capital for the reach during summer low-flow conditions.