Water Resources Research Act Program

Principal Investigators: Katie Fogg

Abstract: The spatially heterogeneous water temperatures of floodplain streams provide areas of thermal refugia tothreatened and endangered Salmonid species [8]. One of the primary mechanisms causing this thermal variation is hyporheic exchange, which is the bidirectional exchange of stream water with the channel's underlying alluvial aquifer [11]. During the hot summer months, upwelling hyporheic water is generally cooler than mean stream temperature, supplying areas of cool water to heat-stressed Salmonids [14]. Streams of the western U.S. are predicted to become warmer into the 21st century [9], therefore cool-water refuge in floodplain reaches may be crucial for the survival of future Salmonid populations. Understanding the heat exchanges driving hyporheic and stream channel temperature is imperative for the successful management and restoration of stream temperature regimes.Floodplain streams reaches have a unique fluvial landscape where conventional management strategies for reducing stream temperature are largely ineffective. The most common strategy for reducing heat input to a stream is planting stream-bank vegetation to increase channel shade [2]. Western floodplain streams have snow-melt dominated hydrology, causing high-energy spring floods which creates an annual scour zone much wider than the summertime base-ow channel [13]. This wide annual scouring pre-vents young vegetation from reaching maturity and thus prevents the establishment of substantial vegetative shade to the base-stream channel. Annual scouring creates a fluvial landscape where vegetation is rarely close to the base-ow channel. Yet, the distant floodplain vegetation may indirectly affect stream channel temperatures by protecting shallow hyporheic water from solar radiation(Figure 1). Prior studies provide indirect evidence for such an effect. Where forests are clear-cut outside of riparian buffer strips, researchers have documented subsequent stream channel warming despite shade from the intact riparian canopy [10, 5]. Such effects are attributed to the heating of shallow subsurface water from the increased solar radiation in the clear-cut beyond the riparian buffer [10, 3]. However, there are no studies that specifically address how variation in floodplain shade might affect hyporheic water temperature and associated stream channel temperature response (Figure 1).