Phase 2 Report

Environmental Flow Needs Assessment in the Okanagan Basin

Environmental Flow Needs Assessment in the Okanagan Basin (ONA 2020)


The need to develop Environmental Flow Needs (EFNs) for Okanagan streams was prioritized with the implementation of the British Columbia Water Sustainability Act in 2016. Subsequently, the Okanagan Basin Water Board (OBWB), Okanagan Nation Alliance (ONA) and B.C. Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNRORD) implemented an EFN-setting project for the Okanagan. Phase I consisted of a collaborative process that developed a robust methodology for EFN setting specific to Okanagan streams, and the acquisition of information needed to customize the methods for each stream. This report, representing Phase II of the Okanagan EFN project, describes the process of determining EFN regimes for 18 tributaries in the Okanagan Basin using the methods developed in Phase I.

Section 1.0 of this report provides a description of the project and study area, and also describes how streams were prioritized for EFN-development. The methodologies applied are presented in Section 2.0. EFNs were recommended for each stream using the desktop-based “Okanagan Tennant method”; additionally the field-based “Okanagan Weighted Usable Width (WUW) method” was used to further refine EFNs for ten of the 18 streams. In addition to the methods outlined in Phase I, this report evaluated the utility of an alternative model-based method called “System for Environmental Flow Analysis” (SEFA). Critical flows were also recommended for each stream using either desktop- or field-based methods depending on data availability.

The Okanagan Tennant method is a modification of the widely used Tennant method, which sets EFNs as a proportion of the Long-term Mean Annual Discharge (LTMAD) required to sustain a given species and life stage (flow standard) in a specific time period (periodicity). Okanagan Tennant EFNs are the lower of the flow standard and the median naturalized flows to ensure that EFNs are realistic and attainable in the context of the natural hydrograph. The field-based Okanagan WUW method utilizes standard WUW approaches that integrate the effect of changes in flow on wetted width, depth, and velocity with habitat suitability indices to calculate the weighted quantity of habitat available for a given species and life stage of fish. The Okanagan WUW method focuses the assessment of flow-related habitat changes within the range of historical or expected flows between the critical flows at the lower and the median naturalized flows at the upper end. Okanagan WUW Analysis was found to be useful for EFN setting particularly in streams with unusual flow patterns or heavily modified channels. The method characterizes local variations in channel and flow conditions that influence fish habitat in greater depth than the Okanagan Tennant method.

Critical flows represent the streamflow below which catastrophic consequences to fish populations may occur. In the absence of field data, critical flows were recommended as a proportion of LTMAD. Where WUW data was available, critical riffle analysis was completed to recommend critical flows that maintain certain minimum depths or widths.

A summary of relevant background information as well as recommended EFNs and critical flows are provided for each stream in Sections 3.1 to 3.18. Further stream-specific data such as maps, descriptions of field sites, hydrometric and water temperature data, WUW curves, detailed weekly EFNs, and percentile flow data are located in Appendices B1-B18. EFNs varied widely between streams depending on stream size, local fish populations, channel and flow conditions. EFNs for spring-spawning species such as Rainbow and Steelhead were generally achievable in most streams due to naturally high flows during freshet that produce near optimum conditions. However, EFNs for most other species that rear or spawn in the summer and fall season were constrained by naturally low flows. Most affected are stream-rearing juvenile fish such as Rainbow, Steelhead and Chinook, as well as early fall spawning species including spring Chinook and some Kokanee stocks. Spring Chinook in particular are extremely vulnerable to low flows due to their large body size, mid-summer migration timing and long holding period. Key spring Chinook tributaries in the southern Okanagan routinely experience very low flows or even dry streambed, likely totally preventing spawner access or success. Later fall spawning species such as Sockeye generally benefit from slightly increased flows following fall rain events, but EFNs are heavily constrained by naturally low flows nonetheless. All of those species and life stages would benefit from flows greater than the EFN which provide increased WUW.

Streamflow datasets required for the EFN analyses were developed for this project by Associated (2019). The EFN setting approach relies heavily on estimated naturalized streamflow data, which is inherently uncertain due to a scarcity of historic and current hydrologic data. Estimation of naturalized and residual (after water use and management) flows is complicated by a lack of accurate water use and diversion information. Most of the study streams have naturally low flows during summer and winter low flow periods. Water use during the summer has noticeable impacts on streamflows in many streams and is an obstacle to meeting EFNs, and in some cases, critical flows. If water storage and releases as well as water use were maximized under current licences, a large number of the study streams would dry up entirely for a large part of the summer. This over-allocation should be addressed in the future.

Section 4.0 contains a review of EFN setting methods and data sources; a summary of EFN and critical flow setting approaches, uncertainties, and recommendations for each stream; as well as a complete list and summary of EFNs and critical flows for all streams. Further, recommendations are made for EFN setting in the Okanagan and in general, and knowledge gaps are identified. The report concludes with an outline of next steps. Key recommendations are: a general call for increased hydrometric monitoring for improved naturalized flow estimation, monitoring of EFN implementation, and to develop a better understanding of flow regulation and water use impacts; field confirmation of several specific EFNs; analysis of existing stream temperature data to inform EFN setting; development of habitat suitability index curves for spring Chinook spawning in small streams; and development of EFNs for anadromous salmonids in Okanagan Lake tributaries with the recent establishment of fish passage.


Syilx/Okanagan communities have always recognized and nurtured a strong connection towards siwɬkʷ (water). The importance of water in Syilx/Okanagan communities and governance is related through captikʷl and the natural laws. Syilx/Okanagan governance systems have always sustainably and respectfully managed water (syilx water declaration

  •   Table 1-2: Criteria used for fieldwork prioritization and methods selection for EFN setting in 18 Okanagan streams

This report documents the process of applying the methods outlined in Phase I (Associated 2016). Methods for establishing Okanagan EFNs were developed through a comprehensive effort that included extensive collaboration with stakeholders and experts, as well as a thorough literature review of EFN setting approaches used locally and elsewhere in North America. The resulting EFN Phase I report (Associated 2016) outlined two primary methods to recommend EFNs for the study streams: an office-based exercise referred to as the “Okanagan Tennant method”, which is a variation of the B.C. Modified Tennant method that was successfully used in the Okanagan in the past; and a field-based, stream-specific method requiring hydrometric and fish habitat data, called the “Okanagan WUW method”. In addition, this report evaluates the utility of an alternative model-based approach called “System for Environmental Flow Analysis” (SEFA) for its ability to provide habitat information for EFN setting where gaps in the field data exist (Section 3.1.1 and Appendix C). Further, a concurrent study on biological indicators (benthic macroinvertebrates) in relation to streamflow conditions provided another alternative approach that was compared to the methods employed in this report (Section 4.3).

Initially, EFNs were determined for all 18 selected streams using the desktop Okanagan Tennant method. EFNs were further refined for 10 of the 18 streams using Okanagan WUW analyses of field data (  Table 1-2). Critical flows were recommended for all streams based on a proportion of flow and further refined, where possible, using field transect data collected for the EFN analysis. The following sections describe the methods for hydrometric data collection, Okanagan Tennant analysis, Okanagan WUW analysis, critical flow analysis, and flow sensitivity assessments.


This section presents recommended EFNs for 18 Okanagan streams developed using the methods described in Section 2.0. Each subsection outlines watershed characteristics relevant to EFN setting and summarizes available literature.

Stream-specific data considered in the EFN setting process is described and the recommended EFNs are presented in summary tables and figures.

Additional stream-specific information is provided in Appendices B1 to B18, including:

  • transect locations, descriptions and photos;
  • habitat mapping;
  • discharge and water temperature records;
  • stream-specific flows and periodicity information;
  • detailed weekly Okanagan Tennant, WUW and final recommended EFNs;
  • WUW curves;
  • critical flow assessments; and
  • percentile flow data.

EFNs were recommended for 18 Okanagan streams using the Okanagan Tennant method and for 10 of those streams, were further refined using the Okanagan WUW method (Associated 2016). These EFNs were developed through an extensive collaborative effort including experts and stakeholders, are robust and realistic in the context of naturally available flows, and are based on the best available information at this time. In addition, critical flows were recommended for all streams based on a proportion of the LTMAD or fish habitat data, where available. The process of applying the EFN setting methods recommended in the Phase I report (Associated 2016) to the 18 streams created a deeper understanding of each stream’s distinct biological, hydrological, and physical characteristics as well as history of human use and modifications, and EFNs were developed under careful consideration of each.

This section provides a review and summary of applying the prescribed methods, recommendations for adjustments, as well as considerations for EFN implementation. Further, a summary of EFNs and critical flows is provided as well as a discussion on data quality and data needs. This section concludes with recommendations for EFN setting initiatives specific to the Okanagan and in general, and a description of next steps.

4.1 Review of EFN Setting Methods and Data Sources

Okanagan Tennant EFNs were to be calculated for each stream as the lower of the highest flow standard or the median naturalized weekly flow for a given time step. They were to be further refined using the WUW information collected in 10 of the streams. Okanagan Tennant EFNs and final recommended EFNs are presented for all 18 streams in Appendices B1-B18. The EFN setting procedure generally followed the methods outlined in the Phase I report (Associated 2016). Refinement of the methods following their application was anticipated in the Phase I report and several adjustments to the methods, as well as sources of uncertainty, are discussed below. Further, stream-specific information on EFN setting methods, uncertainties, considerations for EFN implementation and recommendations are made in  Table 4-1.

  •  Table 4-1: Summary of EFN setting approach, uncertainties and data needs by stream
4.2 Summary of recommended EFNs and Critical Flows   PDFThis section provides a summary of the EFN and critical flow recommendations as well as comments on general patterns observed. Recommended EFNs for the study creeks are provided in   Table 4-2 and critical flows and flow sensitivities are provided in   Table 4-3. Climate change will affect both the timing and magnitude of hydrographs and stream temperatures and the EFNs and critical flows in this report apply only to current climate conditions. They should be reviewed periodically in the future and adjusted, if warranted, to reflect changing climate conditions and any other stream changes or new information.

  •  Table 4-2: Recommended EFNs for the 18 study streams
  •  Table 4-3: Critical flows and flow sensitivities for the 18 study creeks
4.3 Recommendations   PDFThis section contains recommendations specific to this study and the Okanagan as well as for future EFN projects in general. Further, knowledge gaps and potential research topics are discussed.

4.4 Next Steps   PDF

The goal of the Okanagan EFN Project was to produce defensible, transparent and robust EFN values for Okanagan streams. Following completion of this technical exercise, the initial next step is for the larger community to review the EFN and critical flow recommendations for each stream. This will include a review by ONA bands for creeks within their areas of responsibility. The Phase I Report (Associated 2016) should be updated with the changes to the methods described above and any changes identified during the review phase.

Upon agreement on this technical report, there will be a collaborative effort to set final EFNs that balance water demands with ecological needs within a socio-economic context. The focus of this next step will be to identify societal values, and allow for the ability to understand, identify, and make informed decisions as they relate to tradeoffs that exist between EFNs and societal demands (Associated 2016). The undertaking would conclude with the development of an implementation plan. On behalf of ONA, the ONA Natural Resource Council and Chiefs Executive Committee will be engaged in implementation planning with the long term goal of using EFNs for Okanagan water law development.

kʷu‿yʕayʕát iʔ‿kʷu‿sqilxʷ kscpútaʔstm áłiʔ y̓lmixʷmtət iʔ‿siwłkʷ.

Water must be treated with reverence and respect.

áłiʔ íʔ n̓xʷl̓xʷl̓tan̓tət lut kstan̓músmn̓tm, áłiʔ ksctxt̓stim yʕayʕat iʔ‿stim.

Our relationship with water is not taken lightly, we are responsible to ensure that our relation can continue to maintain the health and resiliency of our land and animals.

– Excerpt, Okanagan Water Declaration, July 31, 2014



Appendix A:

Appendix B – Stream specific data:

The following data is presented for each stream where available:

  • Habitat mapping
    • Habitat mapping was completed through the Okanagan Ecosystem Initiative project (Enns et al. 2020)
  • EFN Transect locations
    • Map of EFN transects and hydrometric stations
  • Transect descriptions and photos
  • Discharge records
    • Discharge record at EFN hydrometric stations
    • Historic WSC station discharge
  • Water temperature records
    • Water temperature record at EFN hydrometric stations
  • Flow standards and periodicity – Okanagan Tennant analysis
  • Final EFN and critical flows
    • Comparison of Okanagan Tennant EFNs, WUW EFNs and critical flows
  • Weighted Usable Width curves
  • Critical flows
    • Critical riffle analysis
    • Final critical flows and criteria used
    • 30-day naturalized low flows for summer and winter
  • Percentile flows
    • Comparison of EFN and streamflow percentiles (Figure)
    • Weekly naturalized, residual and maximum licensed percentile flow tables

Appendix C:

Appendix D: