St. Vrain & Left Hand Water Conservancy District

CU Boulder MENV Capstone
SVLH Water Conservancy District

How much water does St. Vrain Creek need?

Amid a 25+ year drought, Colorado water managers have to balance water rights and policy with both human and ecosystem needs. We surveyed St. Vrain Creek reach by reach to answer a practical question for the District: how much water does each stretch actually need to thrive?

Shadia Nagati & Jessica Reid

See the reaches
A surveyor in a high-visibility vest holding a stadia rod at the edge of St. Vrain Creek, with the transect tape strung across the channel

01Storymap

Fourteen reaches, from Lyons to Longmont

We focused on the 32-mile section of St. Vrain Creek between Lyons and Longmont, which supplies water to farms, towns and wildlife. After the 2013 flood, the District built a Stream Management Plan to guide the basin's recovery and long-term stream health; these surveys feed straight into it. The map below shows every reach and transect in the study, and the interactive GIS version ships 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

How we measured it

We searched for appropriate cross-sections to survey transects across 14 segments, or reaches, collecting data in riffles - the shallow, fast sections that dry up first when stream supplies decline. Using bankfull width, water surface slope, depth and velocity, we modeled habitat criteria in R2Cross, an open-access software program.

We completed surveys across 13 of our 14 target reaches; one short reach carried too much backwater flow, with no riffles in which to obtain data. One transect (13A) reached parameters outside R2Cross's capacity and needed extra modeling training from Colorado Water Conservation Board and WRA staff.

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, PhD, 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.

We thank the staff of the Colorado Water Conservation Board and WRA for the additional R2Cross modeling training that made the hardest transects possible.

Finally, we thank the members of the SMP Flow Advisory Working 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, PhD

    Meghan McCarroll, PhD

    MENV Advisor
    University of Colorado Boulder

    Guidance throughout the capstone

03Data

Survey data

Cross-section measurements and modeled 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

Next up: finalize the hydraulic modeling, then present to the SMP Flow Advisory Working Group and at the Sustainable Colorado Watersheds Conference. The report, policy brief and stakeholder whitepaper follow.

05References

Works cited

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 - thirteen reaches across a summer of low water.