CU Boulder MENV Capstone · SVLH Water Conservancy District

How much water does St. Vrain Creek need?

Colorado water law protects whoever claimed the water first, not the creek itself. We surveyed St. Vrain Creek reach by reach to find the minimum flow each stretch needs to stay healthy, and turned it into numbers the District and its partners can act on.

Shadia Nagati & Jessica Reid

See the reaches
Target reaches
14
Cross-sections surveyed
20
Average minimum flow
~16CFS
Habitat criteria to pass
2of 3
St. Vrain Creek running low between grassy banks

01Storymap

Fourteen reaches, from Lyons to Longmont

Each surveyed reach sits somewhere along the creek's descent out of the foothills. The map below shows every reach and transect in the study; the interactive GIS version goes live with the final report.

Figure 1. Environmental flow survey reach details along St. Vrain Creek.
Diagram of the watershed divided into alpine and subalpine, canyons and foothills, transition, and plains
Figure 2. Upper and lower watershed zones along the study area.

Interactive storymap

The ArcGIS storymap will let you walk the creek reach by reach, with photos and flow recommendations at each transect.

Link coming soon

02Acknowledgements

We wish to thank Scott Griebling, Water Resources Engineer at the St. Vrain and Left Hand Water Conservancy District, for his supervision and support during fieldwork and data analysis and Meghan McCarroll, MENV Advisor, for her guidance throughout this project.

We also thank the St. Vrain and Left Hand Water Conservancy District and the University of Colorado Boulder's Masters of the Environment program for their support of this project.

Finally, we thank the members of the SMP Flow Working Advisory Group for their time and input, which will inform the continued development of this work.

  • Scott Griebling

    Scott Griebling

    Water Resources Engineer
    St. Vrain & Left Hand Water Conservancy District

    Supervision through fieldwork and data analysis

  • Meghan McCarroll

    Meghan McCarroll

    MENV Advisor
    University of Colorado Boulder

    Guidance throughout the capstone

  • SMP Flow Advisory Group

    Stakeholder working group
    St. Vrain & Left Hand watershed

    Time and input shaping the recommendations

03Data

Survey data

Cross-section measurements and modelled discharge for every completed transect. Files are published here as each reach clears review.

  • Habitat criteria by reach

    Mean depth, wetted perimeter and velocity discharge for each transect.

    Coming soon

  • Cross-section field sheets

    Raw R2Cross field measurements, by reach and survey date.

    Coming soon

  • Discharge summary

    Average and median discharge meeting habitat criteria across all reaches.

    Coming soon

04Reports

Deliverables

The technical guidance report, policy proposal and stakeholder whitepaper follow the final round of surveys in September.

05Sources

References

The literature, field manuals and District documents behind the method and the recommendations.

  1. Acreman, M. (2016). Environmental flows - basics for novices. WIREs Water, 3(5), 622–628. doi.org/10.1002/wat2.1160
  2. Acreman, M. C., & Dunbar, M. J. (2004). Defining environmental river flow requirements–a review. Hydrology and Earth System Sciences, 8(5), 861–876. doi.org/10.5194/hess-8-861-2004
  3. Amos, A. L. (2006). The use of state instream flow laws for federal lands: Respecting state control while meeting federal purposes. Environmental Law, 36(4), 1237–1283.
  4. Colorado Division of Water Resources. (n.d.). St. Vrain Creek at Lyons discharge data (SVCLYOCO). dwr.state.co.us
  5. Colorado Water Conservation Board. (2023). Colorado Water Plan. coloradogeologicalsurvey.org
  6. Colorado Water Conservation Board, Stream and Lake Protection Section. (2024). R2Cross field manual. dnrweblink.state.co.us
  7. Griebling, S. (2022). Presentation on the St. Vrain and Left Hand Stream Management Plan [Video]. YouTube. youtu.be/pD5qwy-LKh8
  8. Kendy, E., Apse, C., & Blann, K. (2012). A practical guide to environmental flows for policy and planning. The Nature Conservancy.
  9. Mason, S. (n.d.). R2Cross overview [Video]. River Network Peer Learning Network.
  10. Oikonomou, P. D., Sidhu, R., Wible, T., & Morrison, R. R. (2021). R2Cross: A web-based decision support tool for instream flows. JAWRA, 57(4), 652–660. doi.org/10.1111/1752-1688.12939
  11. Poff, N. L., Allan, J. D., Bain, M. B., Karr, J. R., Prestegaard, K. L., Richter, B. D., Sparks, R. E., & Stromberg, J. C. (1997). The natural flow regime. BioScience, 47(11), 769–784.
  12. Poff, N. L., Richter, B. D., Arthington, A. H., Bunn, S. E., Naiman, R. J., Apse, C., Kendy, E., et al. (2010). The ecological limits of hydrologic alteration (ELOHA). Freshwater Biology, 55(1), 147–170. doi.org/10.1111/j.1365-2427.2009.02204.x
  13. River Network. (2021). Stream management plans in Colorado: Progress at five years. CWCB Watershed & Flood Protection Section.
  14. River Network. (2022, November 9). Stream management plans 101 [Video]. YouTube. youtube.com
  15. River Network. (2023). Flow recommendations in SMPs – Opportunity analysis. River Network & CWCB.
  16. River Network & Colorado Water Conservation Board. (2019). St. Vrain and Left Hand Stream Management Plan [Case study summary]. coloradosmp.org
  17. River Network & Colorado Water Conservation Board. (2024). Watershed planning prioritization tool [Interactive map]. coloradosmp.org
  18. St. Vrain & Left Hand Water Conservancy District. (2020). Stream management plan: Phase I summary report. svlh.gov
  19. St. Vrain & Left Hand Water Conservancy District. (2024). Stream management plan: Phase I progress update. svlh.gov
  20. St. Vrain & Left Hand Water Conservancy District. (2025). SMP implementation priority initiatives report.
  21. St. Vrain & Left Hand Water Conservancy District. (2025). District Water Plan: 2024 progress update.
  22. St. Vrain & Left Hand Water Conservancy District. (2025). Environmental flow surveys for St. Vrain Creek [MENV capstone project proposal].
  23. U.S. Environmental Protection Agency. (n.d.). Definition and characteristics of low flows. epa.gov
  24. Wineland, S. M., Başağaoğlu, H., Fleming, J., Friedman, J., Garza-Diaz, L., Kellogg, W., … Wootten, A. M. (2022). The environmental flows implementation challenge. WIREs Water, 9(1), e1565. doi.org/10.1002/wat2.1565
  25. Zellmer, S. B., & Amos, A. L. (2026). Water law in a nutshell (7th ed.). West Academic Publishing.

06Gallery

In the field

Wading, measuring, and re-measuring - twenty cross-sections across a summer of low water.