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1. Hydrologic interactions between streamsand their subsurface hyporheic zones

2. CENTRAL QUESTIONWhat is the hyporheic zone? It derives from the Greek meaning under (hypo) the current or flow (rheo).

3. The hyporheic zone is a portion of the groundwater interface in streams where a mixture of surface water & groundwater can be found (Bencala 1993)

4. The hyporheic zone In the bed and banks of streams, water and solutes can exchange in both directions across the streambed. This process, termed hyporheic exchange, creates subsurface environments that have variable proportions of water from ground water and surface water. Figure from Winter et al. 2000

5. The hyporheic zone The bi-directional exchange of water mixes water & solutes from surface and subsurface environments. An empirical perspective from hydrologists recognizes a hyporheic zone as the subsurface zone receiving at least 10% of water by input from the stream (>10% channel water). Definition after Triska et al. 1989. Figure from Winter et al. 2000

6. Some important concepts Length and timescale of the interaction help to distinguish hyporheic exchange from the much larger (and longer term) channel and groundwater interactions Figure from Winter et al. 2000

7. Some important concepts • Hyporheic flowpaths leave and return to the stream many times within a single study reach; the exchange of surface water back and forth between the active channel and the subsurface is rapid. Within several kilometers, stream water in relatively small channels is often completely exchanged with the porewater of the hyporheic zone • This repeatedly brings stream water into close contact with geochemically and microbially active sediment. Figure from Harvey & Wagner 2000

8. Chemical gradients at the interface Steep concentration gradients in dissolved constituents exist at the groundwater/surface water interface, due to mixing of chemically-different waters and chemical transformations

9. Zonation of metabolic activity in hyporheic zone • Hydrologic exchange regulates sources of oxygen, carbon, & nutrients to organisms and plants • This directly controls trophic structure and primary productivity in streams

10. Hydrologic fluxes potentially can enhance ecosystem production on both sides of the groundwater/surface water interface, by supplying solutes that are needed in biological metabolism.

11. The term hyporheic zone was originally used by Orghidan (1959), who described the interface as a new groundwater environment containing distinctive biota. Higher than expected abundances of aquatic insects were found in sediments where concentrations of oxygen were high. hyporheic zone is important to aquatic biota

12. hyporheic zone can be a source of nutrients:example from Sycamore Creek, Alaska In a study of the ecology of Sycamore Creek near Phoenix Arizona, hydrologic exchange fosters recovery of desert stream ecosystems following storms, by stimulating recycling of the nutrients from buried organic matter.

13. hyporheic zone at Sycamore Creek, Arizona Photo courtesy of Stuart Fisher, Arizona State University

14. Following a storm, oxygen advection into hyporheic zone stimulates aerobic decomposition of buried algae. Released nutrients are delivered to the surface channel by return flows from hyporheic flow paths. Nitrate in surface channel stimulates new primary productivity by algae. After Valett et al. 1994

15. Abundance of algae in streambed sediments, as indicated by concentration of chlorophyll a, was markedly greater in areas of upwelling (where groundwater moved upward through the sediments) than in areas of downwelling. After Valett et al. 1994

16. a Active Layer Permafrost Permafrost Permafrost Hyporheic zone: under and adjacent to stream with active exchange of water

17. In dry valley streams the “hyporheic zone” is observed as a wetted zone adjacent to stream

18. Dry valley streams flow through porous alluvium with high rates of “hyporheic exchange”.

19. Experimental enrichment with nitrate and phosphate: 1.5 hrs

20. Sampling Green Creek during experiment

21. Stream algal mats win! Added nutrients are taken up and do not reach the lake.

22. hyporheic zone feedbacks to terrestrial landscape:example from southeast Alaska • Pacific salmon spend most of their lives growing at sea before returning to fresh water to spawn and die in their natal streams, carrying marine-derived nitrogen in their body tissues. Returning salmon provide a seasonal food source for birds & mammals, and nutrients from decaying salmon carcasses are incorporated into freshwater biota at various trophic levels. • From this nutrient subsidy, growth rates are significantly increased in trees near spawning streams. • MDN be transferred from spawning streams to riparian forests by flooding which deposits salmon carcasses on stream banks & through the transfer of dissolved nutrients from spawning by hyporheic exchange.

23. Shading DOC. inputs Sediment, nutrient filtration Bank stabilization Flooding Bank erosion Sediment deposition Hyporheic MDN fertilization of riparian vegetation Enhanced riparian growth Changes in spp. composition, diversity, soil chemistry Materials carried downstream MDN carried upstream Slide courtesy of Bob Naiman, University of Washington

24. Where does hyporheic exchange take place?example from southwest Alaska • Lateral nutrient transfers occur in the hyporheic zone & in adjacent riparian floodplains along spawning reaches. • Hyporheic storage & re-release of MDN is an important mechanism by which nutrients are retained over winter within stream ecosystems and subsequently made available to primary producers the following growing season.

25. hydraulic head gradients in a well network Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington

26. DO gradients illustrating hyporheic exchange Slide courtesy of Tom O’Keefe & Rick Edwards, University of Washington

27. Photo courtesy of Bob Naiman, University of Washington

28. Loss of water from stream reach to the hyporheic zone commonly related to meanders in streambed Figure from Winter et al. 2000

29. A study of a third order gravel bed stream in the Colorado Rocky Mountains, characterized by pools & riffles, indicates geomorphic controls on where hyporheic exchange takes place. Where does hyporheic exchange take place? example from St. Kevin Gulch, CO

30. Inflow of water from the hyporheic zone to the stream was greatest at the downstream end of riffles. After Harvey & Bencala, 1993

31. Stream water flows into the subsurface beneath and to the side of steep sections of streams (riffles), & ground water enters streams most readily at the upstream end of deep pools From Harvey & Bencala, 1993

32. important points about controls on where hyporheic exchange takes place • Hyporheic exchange caused to a large extent by the irregular topography of the streambed, which creates pools & riffles characteristic of mountain streams. • Channel irregularity is an important control on the location of groundwater inflow to streams and on the size of the hyporheic zone in mountain streams because changes in slope determine the length & depth of hyporheic flow paths. Harvey & Bencala, 1993

33. Loss of water from stream reach to the hyporheic zone commonly related to abrupt changes in slope of the streambed Figure from Winter et al. 2000

34. How extensive can a hyporheic zone be? • Depending on the type of sediment in the streambed and banks, the variability in slope of the streambed, and the hydraulic gradients in the adjacent ground-water system, the hyporheic zone can be as much as several feet in depth and hundreds of feet in width. • The dimensions of the hyporheic zone generally increase with increasing width of the stream and permeability of streambed sediments.

35. Example of large hyporheic zone: Willamette River, OR • A large, 9th order stream with permeable, coarse-grained, alluvial deposits in the channel & floodplain • HZ width = hundreds of m • Nitrogen coming in from regional groundwater was removed in the HZ • HZ gains/losses in flow over reaches of 1-2 km were on the order of 5% of streamflow

36. CENTRAL QUESTIONWhat are some common methods used to quantify hyporheic flowpaths?

37. Water table Unconfined aquifer Subsurface measurements in piezometers Water levels in piezometers are located at the water table. This level is a measure of the hydraulic head, or potential at that point.

38. upwelling & downwelling can be inferred with piezometer monitoring data Vertical flow direction Horizontal flow direction

39. Hydrometric measurements at Rio Calavares, NM Photo courtesy of Michelle Baker, Utah State University

40. Injection & transport of a solute tracer in the stream A tracer is released, and measurements of its passage are made at a location downstream. What will a graph of tracer concentration over time look like at the downstream monitoring point?

41. Injection & transport of a solute tracer in the stream The concentration will rise, reach a plateau, and then decline as the pulse passes the point of monitoring. The solute disperses from its point of release due to the force of the current (advection) and diffusion and turbulent mixing throughout the stream.

43. Dye tracer illustrates water storage at channel margins: 1) sides are initially dye-free Photo courtesy of Jud Harvey, USGS, Reston, VA

44. Dye tracer illustrates water storage at channel margins:2) all parts of channel have dye Photo courtesy of Jud Harvey, USGS, Reston, VA

45. Dye tracer illustrates water storage at channel margins: 3) sides retain dye longer than center channel Photo courtesy of Jud Harvey, USGS, Reston, VA

46. Transient Storage: the temporary retention of solutes in slowly moving or stationary water, and the eventual movement of solutes and water back into the stream channel. Five storage zones observed in a small mountain stream (Bencala & Walters 1983): • turbulent eddies generated by large-scale bottom irregularities • large but slowly moving recirculating zones along the sides of pools • small but rapidly recirculating zones behind flow obstructions, especially in riffles • side pockets • flow in and out of beds of coarse substrate.

47. modelling the injection & transport of a solute tracer in the stream As a first approximation, this curve can be achieved with a basic equation describing advection & dispersion, taking into account stream dimensions & water velocity. Change in solute concentration over time = ∂ C / ∂ t = - U ∂ C / ∂ x + D ∂2 C / ∂ x2 • 1st term describes downstream advection and is proportional to water velocity, u. • 2nd term describes mixing of the solute randomly throughout the mass according to a dispersion coefficient D

48. modelling the injection & transport of a solute tracer in the stream More complicated models are needed to account for additional variables such as groundwater and tributary inputs, channel storage and subsurface flow. Inclusion of terms for transient storage is necessary for the description of solute dynamics in small streams and their hyporheic zones. OTIS: One-dimensional Transport with Inflow and Storage—Conceptual Model (OTIS, Runkel 1998)Represents stream transport, inflow, outflow, & exchange

49. advection dispersion exchange exchange modelling the injection & transport of a solute tracer in the stream Passage of a the tracer, including transient storage is given by: • ∂ C/ ∂ t = -U ∂ C/ ∂ x + D ∂2/C/ ∂ x2 + α(Cs - C) • ∂ Cs/ ∂ t = - α A/As (Cs – C) Where As is the cross-sectional area of a hypothetical storage zone. The rate of dispersion of solute in or out of this zone is proportional to the difference between solute concentration in the storage zone (Cs) and the water column (C) and an exchange coefficient (α).

50. Adding a transient storage term permits the model to account for significant features of the of a solute pulse that the earlier equation is unable to mimic. Specifically, measured passage of a tracer pulse usually shows the rising shoulder of the actual pulse to be more gradual and the descending tail to be prolonged relative to the symmetrical curve generated by the earlier equation.

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