1. Monitoring of Groundwater-Surface Water Interactions in Support of Restoration of Hyporheic Processes in an Urban Stream, Thornton Creek, Washington GSA 2009 Annual Meeting, Portland, Oregon, Session No. 177, October 20, 2009 Paul D. Bakke 1, Katherine Lynch 2, Tracy Leavy 1 and Roger Peters 1 1 Washington Fish & Wildlife Office, U.S. Fish & Wildlife Service, 510 Desmond Drive SE, Suite 102, Lacey, WA 98503 2 Seattle Public Utilities, 700 5 th Ave., Suite 4400, P.O. Box 34018, Seattle, WA 98124 THE CHALLENGE OF RESTORING URBAN STREAMS: At least three forms of degradation are present: high stormwater runoff rates, loss of habitat structure, & degraded water quality Restoration of water quality is poorly documented Many confounding influences Expensive real estate and limited available space for projects PURPOSE OF THIS STUDY: Demonstrate methodology for monitoring restoration of hyporheic processes Conduct pre-project monitoring to establish baseline conditions & inform restoration design METHODS: Stream-segment scale (hydro-geomorphic context): Piezometer approximately 1.4 meters into streambed, measures: piezometric head streambed temperature at 3 elevations VS2DHI Hydraulic-thermal model calibration to estimate Hydraulic conductivity of streambed Groundwater influx rate Heat flux into/out of streambed METHODS: Habitat-unit scale (local hyporheic interchange) Extensive streambed temperature measurements during one day, late summer base flow season Array of 50 – 60 tubes, 10 cm into gravel. Temperature in each tube measured 3 times. Compare intra-gravel temperature (bottom of tube) with surface water temperature to map zones of upwelling & downwelling, & relative strength THORNTON CREEK WATERSHED CHARACTERISTICS: Watershed area: Total 28.4 sq. km (11.0 sq. mi.) North Fork 16.7 sq. km (6.5 sq. mi.) South Fork 9.1 sq. km (3.5 sq. mi.) Current (1999 - 2008) mean annual flow: 0.261 m 3 /s (261 L/s or 9.2 CFS) Human population: 75,400 (estimated) Proportion of watershed with impervious surfaces: 49 percent Concept No. 1: Alluvial fan - Step-pool channel (Rosgen A or G) with hyporheic streambed Advantages: Fits well into small available area Passes bedload sediment well Streambed scour could prevent clogging by fines Disadvantages: Hyporheos confined to channel footprint Little active floodplain function or floodplain hyporheic component Difficult to prevent hyporheic water from being lost to water table without greatly restricting extent of subsurface flow Streambed scour potential high Concept No. 3: Depositional forested wetland complex with complex subsurface interactions Advantages: Fits well into small available area Consistent with historic habitat conditions at site Wetland function creates nutrient sink for water quality improvement, especially if inundation is seasonal, with vertical movement of anaerobic zone Habitat for wetland species, including beaver Disadvantages: Does not pass sediment = long term aggradation likely without active sediment removal Sluggish wetland flow & low hydraulic conductivity reduces hyporheic exchange, with loss of cooling potential RESTORATION OF HYPORHEIC PROCESSES at Forks Confluence site: Goals - Sustainable water quality improvement (cooling, nutrient reduction) Increased intra-gravel flow for improved spawning habitat 8 cm 1.4 to 1.5 m Streambed Water Locking Cap 3.8 cm diam. steel pipe 3 mm holes South Fork Main Stem North Fork South Fork 1. 2. 5. Concept No. 2: Depositional response reach with single-thread channel (Rosgen C or Bc) plus side channels, floodplain hyporheic pathways Advantages: Active floodplain with side channels Subsurface preferential flow pathways = extensive hyporheos with diverse water residence time Soil water & sediment recharge during moderate floods Traps fine sediment on floodplain More stable fish habitat (less scour, deeper alluvium) Disadvantages: Difficult to fit into available area May aggrade without active sediment removal if sediment sources not controlled

2. RESULTS: Shown for each site - Streambed vertical thermal profile for typical late-summer period, 48 hours long Plan view map of reach with surface-subsurface temperature difference isotherms, showing relative downwelling/upwelling Average piezometric gradient Groundwater influx rate & total reach-scale groundwater input to compare with surface water discharge Crossover dates: the time when the vertical thermal gradient in the streambed changes from positive (streambed cools the surface water) to negative (streambed warms the surface water) Estimated heat flux (cooling or warming) by streambed, with estimated cooling load in o C Map Lake Washington To 95th Street Treatment Reach From Forks Confluence Reach To Lake Washington CONCLUSIONS: Hydraulic conductivities at all sites, even those with alluvial subsurface, is very low Consequently, vertical groundwater input rate is very low at all sites, even though piezometric gradient is large Heat exchange with streambed is currently conduction dominated, so cooling load is low Subtle hyporheic exchange is evident, but extremely slow due to impermeable sediments  Subsurface warmer than surface at two sites (KNK-T, KNK-C, in figure above), indicating substantial lack of advection flow  Lateral groundwater input evident at 2 sites (KNK-T, SFC-C, in figure above) Crossover dates and duration vary widely by site. Inter-annual variability also can be large. Streambed: gravel surface over sand & gravel alluvium Streambed: veneer of alluvial gravel over massive clay Streambed: veneer of alluvial gravel over massive pebbly clay Streambed: gravel surface over sand & gravel alluvium 3. 4. Distribution of temperature differences at each site during one-day extensive study. Crossover period - the time during which the vertical thermal gradient in the streambed changes from positive (streambed cools the surface water) to negative (streambed warms the surface water) References: Conant, B., Jr. 2004. Delineating and quantifying groundwater discharge zones using streambed temperatures. Ground Water, 42(2):243-257. Hsieh, P. A., W. L. Wingle, and R. W. Healy, 2000. VS2DI—A Graphical Software Package for Simulating Fluid Flow and Solute or Energy Transport in Variably Saturated Porous Media. USGS Water-Resources Investigations Report 99-4130. 16 pp. Stonestrom, D.A., and Constantz, J., eds., Heat as a tool for studying the movement of ground water near streams: USGS Circular 1260. 96 pp.