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  • Baseflow is necessary in streams for maintaining trout populations.

  • Declining rainfall and snowmelt, however, due to climate change, are reducing it.

  • In-stream structures could augment baseflow, slightly, by increasing streambed infiltration.

  • More valuable, however, as a result of structures is increased trout refuge above from the raised water levels and directly below from scouring, that is, from creation of plunge pools.

  • Key is backing up water vertically within the channel, not laterally above it, because expanding outside the banks increases water surface area and loss from evapotranspiration.

  • Adding water back-up in tributaries in this way would not decrease total flow reaching the river.

  A. Considerations​​

  • Inflow of subsurface water accounts for baseflow in streams, which is decreasing due to dewatering.

  • Structures installed across streams may augment baseflow.

  • Water may rise to the height of the structures, overtop them, and continue downstream, all within the channel.

  • The important detail for baseflow is that the increased height of water behind the structures will raise water levels just downstream from them.

  • This is not because of water spilling over the structures.

  • Occurring from that is simply greater momentum that increases scouring directly below them.

  • For this baseflow matter, notable, instead, is that the elevated water levels behind the structures increase the pressure of water that moves through the streambed for release upwardly below the structures.

  • The result is a raising of the height of the water directly downstream of structures, that is, a localized augmentation of baseflow.

  • The augmentation is expected to be slight and to fade over only a short distance below structures because the hydraulic conductivity of the streambed likely is small.

  • That is, reasonably, the pressure in the streambed from the elevated water just upstream of the structure will dissipate downstream.

  • The dissipation likely will be complete relatively nearby the structure due to friction as the water flows through the low-conductivity streambed media.

  • The water level above a structure may fluctuate, but the upward pressure of water in the streambed below the structure always will be greater than without a structure.

  A. Considerations, cont.

  • So there will be a small, consistent, localized augmentation of baseflow directly downstream of a structure.

  • Structures across streams and water back-up occur naturally from fallen trees, for example, and from collections of woody debris and rocks placed by heavy stream flows.

  • More augmentation in total can be produced by increasing the number of in-stream structures along the length of a stream channel.

  • In summary, likely, the main benefit from installed structures is more trout refuge due to increased water depth directly above and below them.

  • But, also, there can be expected a slight augmentation in baseflow and a slight reduction in water loss from evapotranspiration as water is directed through the streambed, which is away from air contact.

  B. Preferences

  • A preferred location results in a body of water that is narrower, deeper, and longer than at other sites.

  • That is, narrower to minimize surface area and evapotranspiration, deeper for pressure to increase infiltration, longer for more deep water to infiltrate.

  • Installations should keep raised water levels below bank heights so as not to increase water surface area and resulting evapotranspiration.

Additional reading

  1. Ponce, V. M. and D. C. Lindquist, 1990, "Management of Baseflow Augmentation: A Review," Water Resources Bulletin, American Water Works Association, v. 26, n. 2, pp. 259-268.

  2. DeBano, L. F. and B. H. Heede, 1987, "Enhancement of Riparian Ecosystems with Channel Structures, Water Resources Bulletin, American Water Works Association, v. 23, n. 3, pp. 463-470.

  3. Wheaton, J. M., S. N. Bennett, N. Bouwes, J. D. Maestas, and S. M. Shahverdian, 2019, "Low-Tech, Process-Based Restoration of Riverscapes, Design Manual," Utah State University Restoration Consortium, Department of Water Sciences, Logan, UT.

  4. Pollock, M. M., T. J. Beechie, J. M. Wheaton, C. E. Jordan, N. Bouwes, N. Weber, and C. Volk, 2014, "Using Beaver Dams to Restore Incised Stream Systems," Bioscience, Advance Access Publication, http://bioscience.oxfordjournals.org.

  5. https://www.sciencenews.org/article/stream-survival-beaver-dam-simple-structures-wildfires-drought/amp

  6. Goldfarb, B., 2018, “Beaver Dams without Beavers? Artificial Logjams are a Popular but Controversial Restoration Tool,” Science, AAAS, ScienceMag.org, https://www.science.org/content/article/beaver-dams-without-beavers-artificial-logjams-are-popular-controversial-restoration.

  7. Davee, R., H. Gosnell, and S. Charnley, 2019, "Using Beaver Dam Analogues for Fish and Wildlife Recovery on Public and Private Rangelands in Eastern Oregon," USDA, Pacific Northwest Research Station, Research Paper PWN-RP-612.

  8. Peterson, C., 2018, "Once Considered the Scourge of Agriculture in the West, Ranchers are Now Building Beaver Dams, and Welcoming the Creatures Home," Casper Star Tribune, https://trib.com/outdoors/once-considered-the-scourge-of-agriculture-in-the-west-ranchers-are-now-building-beaver-dams/article_250b5cdb-2b49-5da0-86ab-a7784ba8d570.html.

  9. Axness, D. S. and K. Clarkin, 2013, "Planning and Layout of Small Stream Diversions," U.S. Department of Agriculture, Forest Service, National Technology and Development Program, 2500, Watershed, Soil & Air Management, 1325 1801, SDTDC.

  10. Batlle-Aquilar, J, and P. G. Cook, 2012, "Transient Infiltration Through Ephemeral Streams: A Field Experiment at the Reach Scale," Water Resources Research, v. 48, W11518, DOI:10.1029/2012WR012009.

  11. Bruen, M. P. and Y. Z. Osman, 2004, "Sensitivity of Stream-Aquifer Seepage to Spatial Variability of the Saturated Hydraulic Conductivity of the Aquifer,"  Journal of Hydrology, 293(1–4), 289–302.

  12. Kalbus, E., C. Schmidt, J. W. Molson, F. Reinstorf, and M. Schirmer, 2009, "Influence of Aquifer and Streambed Heterogeneity on the Distribution of Groundwater Discharge," Hydrology and Earth Systems Science, 13 (1), 69–77.

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