Phase 1 and 2 Summary Report

Okanagan Environmental Flow Needs Project - Phase 1 and 2 Summary Report (Associated 2020)

Okanagan Environmental Flow Needs Project – Phase 1 and 2 Summary Report (Associated 2020)

The Okanagan region of B.C. is relatively arid, yet a large number of licences have been issued for the withdrawal of water from creeks, rivers, and lakes for municipal, agricultural, and other purposes. Until 2016, when the B.C. Water Sustainability Act (WSA) came into force, there was little or no legal protection for the aquatic ecosystems of B.C. streams. However, the WSA now requires that decisions on water licence applications consider the streamflows required to sustain aquatic ecosystems. This requirement applies not only to applications for water withdrawals from streams, but to water withdrawals from nearby aquifers that are likely connected to streams. These ecosystem-sustaining streamflows are referred to as Environmental Flow Needs (EFNs), and are formally defined in the WSA as “the volume and timing of water flow required for the proper functioning of the aquatic ecosystem of the stream”. The Critical Environmental Flow Threshold (CEFT) is another parameter defined by the WSA – it is “the volume of water flow below which significant or irreversible harm to the aquatic ecosystem of the stream is likely to occur.” The CEFT is used by the Province to manage water withdrawals during low flow periods.

Previous work has been completed to estimate the EFNs and critical low flows of Okanagan streams, but no previous study has had the necessary rigour or granularity to support well-informed water licensing decisions, or operational water management decisions in the Okanagan Basin.

Climate change is affecting Okanagan streams, and the effects have been noticeable in recent decades. The spring freshet period is starting and finishing earlier, and late summer flows are becoming lower. In addition, summers are becoming longer, hotter, and drier, which is tending to increase demand for water by municipal and agricultural users. These trends both act in the same direction – increasing the stress on aquatic ecosystems – and are expected to continue for the foreseeable future.

In 2015, in response to these issues, the Okanagan Basin Water Board (OBWB), the Okanagan Nation Alliance (ONA), and the provincial government, represented by the B.C. Ministry of Forests, Lands, Natural Resource Operations and Rural Development (FLNR), initiated the Okanagan EFN project (“the project”). Its goal is to determine the EFNs of each major stream in the Okanagan Basin. To date, the project has established EFNs and CEFT values in a subset of Okanagan streams (Figure 1-1). These particular streams were selected for the project in a collaborative analysis by the ONA, FLNR, and OBWB, based mainly on their fish habitat value and current water use pressures.

The present document (the Phase 1 and 2 Summary Report) was prepared by Associated Environmental Consultants Inc. (Associated), and summarizes the work completed to date on the project. It includes the key project reports as Appendices in electronic format, and provides links to the original reports. This document also describes next steps in the project.


Two phases of the project have been completed:

  • Phase 1: Development of methods to determine EFN flow regimes in Okanagan streams.
  • Phase 2: Determination of EFN flow regimes (including CEFT values) for key streams in the Okanagan.

Reports have been produced for each of these two phases. The Phase 1 report (Associated 2016) is titled “Collaborative Development of Methods to Set Environmental Flow Needs in Okanagan Streams.” That report recommends use of a desktop EFN-setting method (referred to as the Okanagan Tennant method) and a field-based EFN-setting method (referred to as the Okanagan Weighted Usable Width [WUW] Method) and is reproduced in Appendix A of this summary report.

The Phase 2 report (ONA 2020) is titled “Environmental Flow Needs Assessment in the Okanagan Basin” and is reproduced in Appendix D of this summary report.

Other work has been completed in support of these two project phases. The Okanagan Tennant method relies on streamflow information. Accordingly, the OBWB commissioned a report titled “Recommended Methods for the Development of Streamflow Datasets to Support the Application of the Okanagan Tennant Method in Okanagan Streams.” This report is listed in the reference list as Associated (2017) and is reproduced as Appendix B of this summary report. A follow-up study titled “Streamflow Datasets to Support the Application of the Okanagan Tennant Methods in Priority Okanagan Streams” used the methods recommended by Associated (2017) to create the streamflow datasets that became the foundation of the EFN-setting work completed during Phase 2. That study report (Associated 2019) is reproduced in Appendix C of this summary report.

Finally, it became evident during Phase 2 that Okanagan streamflows have likely been influenced by climate change in the recent past. A technical memorandum (reproduced in Appendix C of Associated 2019) explains this issue and the methods adopted to account for it.


This section summarizes the work completed in Phase 1 of the project, which is described in detail in the Phase 1 report, reproduced in Appendix A.

The main objective of Phase 1 was to develop defensible, transparent, and robust methods for determining the EFNs of Okanagan streams. The Phase 1 report recommends two EFN-setting methods, and provides information needed to customize the methods for each of the 19 key streams shown in Figure 1-1. The work was completed by a large group of aquatic scientists representing a broad range of expertise and experience, and a large number of aquatic scientists from several levels of government provided input during the work and reviewed the finished product. The Phase 1 report reflects this broad level of engagement, contribution, and consensus.

The Phase 1 report recommends a desktop EFN-setting method for low-risk streams followed by a field-based method for higher risk streams. The desktop method is referred to as the Okanagan Tennant method, and is a variation of the B.C.-modified Tennant method that has been successfully used in the Okanagan Basin. The field-based method is also a variation of a method that has been successfully used in the Okanagan Basin, and is referred to as the Okanagan Weighted Usable Width (WUW) method. Both methods are displayed on large format flowcharts (Figures 6-1 and 6-4 of the Phase 1 report) and fully described in the Phase 1 report. They are described as “moderately prescriptive guidelines”, which suggests that there is flexibility in the level of effort to apply at each step of the flowcharts, and that experience and good judgment are required to apply the methods. This is particularly applicable to the use of the Okanagan WUW method; factors such as the number of species present, the value of the aquatic habitat of the stream, and risk tolerances will determine the appropriate level of effort to apply to a particular stream.

The Okanagan Tennant method uses streamflow information to set EFNs. The basis for the method is either natural or naturalized (i.e. estimated natural) streamflows for the reaches occupied by key species. The term ‘natural’ is used to describe a streamflow record unaffected by human influences. When a streamflow record is affected by human influences, natural streamflow conditions must be estimated by identifying and removing the human influence. The resulting flow record is referred to as ‘naturalized’.

The concept underlying the use of natural (or naturalized) streamflows as a reasonable basis for EFN-setting is that key aquatic species have become adapted to natural conditions over many centuries before direct modern anthropogenic influences such as reservoir management and water withdrawals, and indirect influences such as climate change began to affect the natural flow regimes of Okanagan streams.

The Okanagan Tennant method is intended for developing an initial understanding of the risks to aquatic habitat and ecological processes from existing and proposed water allocations relative to natural or naturalized streamflows. The method is designed to set an EFN streamflow regime that meets the WSA definition of a properly functioning ecosystem. It recommends a monthly time step for August through March and a weekly time step for April through July. EFNs should be set at the lower of the median naturalized streamflow for the time period of interest and the “instream presumptive flow standard”, which is a value that has been previously estimated based on relevant similar conditions (in the absence of site-specific EFN assessments) as the portion of long-term mean annual discharge (LTMAD) required to sustain a given ecosystem, species, and life stage. Henceforth in this summary report, this value is referred to simply as the “flow standard.”

The Phase 1 report provides guidance for understanding the implications of flows lower than the recommended EFNs. It acknowledges that flows in dry years will drop below EFN values, and recommends, for real-time operational management purposes (not for water licensing purposes), allowing the EFN to drop to match the natural low flow.

The Okanagan WUW method extends the Okanagan Tennant method to consider site-specific fish and fish habitat conditions in the streams and reaches of interest, and refine the EFNs recommended using the Okanagan Tennant method. The WUW is calculated using depth and velocity measurements at intervals along transects located in the appropriate habitat units for the species and life stages of interest, in conjunction with Habitat Suitability Index (HSI) curves. The Okanagan WUW method addresses the tendency to recommend optimal flows by recommending that streamflows be scaled between zero and one, where “zero” is defined as the critical environmental flow threshold (CEFT) (i.e., a flow below which severe consequences to aquatic populations are expected), and “one” is defined as the median (or suitable alternative based on stream-specific considerations) flow for the particular time period. The report provides guidance for using the information collected to judge the risks associated with flows less than EFNs. (However, during Phase 2, this WUW scaling calculation was not considered necessary for EFN-setting, as explained later in this summary report, but its usefulness for judging risks related to flows lower than EFNs is acknowledged.)

The Phase 1 report advises collecting additional relevant data on the aquatic ecosystem to inform EFN-setting. It also provides guidance on estimating CEFT values (including methods to estimate riffle passage flows), but does not provide a prescriptive approach to estimating CEFT. Finally the report recommends specifying ecological function flows.

The report includes several appendices, including one that provides information and data relevant to EFN-setting for each of 19 key streams. As noted earlier, these streams were selected collaboratively by the ONA, FLNR, and OBWB from the full set of Okanagan tributary streams, based mainly on their fish habitat value and current water use pressures.

The Phase 1 report cover identifies it as a “Working Document, Version One”, which conveys the intention of the project partners (OBWB, ONA, and the Province of B.C.) that the methods will be revised and updated based on experience gained through their use. Additionally, it was intended that the report be reviewed by ONA aquatic scientists and knowledge keepers and that additional information would be added. This contribution occurred during Phase 2 of the project, which was led by the ONA.


This section of the summary report summarizes the work conducted during Phase 2 of the project:

  • Development of flow naturalization methods (i.e., methods for estimating natural flow when flow records are confounded by human influences) to create several streamflow datasets for use with the Okanagan Tennant method (Section 4.1);
  • Creation of streamflow datasets for each of 18 streams selected for analysis in Phase 2 (Section 4.2); and
  • Development of EFNs using the Okanagan Tennant method for each of the 18 Phase 2 streams, and using the Okanagan WUW method for 10 of those streams, and setting the Critical Environmental Flow Threshold (CEFT) for all 18 streams (Section 4.3).

The Phase 2 work was led by a project leadership team consisting of staff from the OBWB, ONA, and FLNR.

  • Streamflow Naturalization Methods
  • Streamflow Datasets for each Priority Stream
  • Overview of Phase 2 Report

The Okanagan Environmental Flow Needs Project has produced the most comprehensive estimates of streamflow conditions and both desktop and field-based EFN recommendations ever assembled for the Okanagan Basin. The EFN recommendations are credible, as they have been developed using standard methods that have been collaboratively customized for the Okanagan. The EFN recommendations have a strong scientific foundation, and were developed by a leadership team of senior scientific staff of ONA, OBWB, and FLNR. ONA staff managed and conducted the EFN-setting work.

Key learnings of the Phase 2 work

  • Human intervention has physically modified many Okanagan streams and their streamflows, generally causing reduced flows in the critical summer and early fall period, but in some cases causing increased flows at this time of year.

  • Use of the full currently licensed water allocations would dry up many Okanagan creeks in summer and fall.

  • Higher levels of government have historically provided insufficient resources for monitoring streamflows, water diversions, and fish habitat conditions.

  • Flow naturalization is uncertain where insufficient data on streamflows and water withdrawals exist. In data-poor streams, naturalized flow estimates may lack the accuracy and precision needed for EFN-setting. In these cases, the field-based WUW approach should be used for EFN-setting.

  • The Okanagan Tennant EFN-setting approach has been judged to be valid for the Okanagan, based on field observations of fish habitat conditions over a range of flows in 10 of the 18 streams where Okanagan Tennant EFN analysis had been applied. However, the Okanagan WUW EFN-setting approach provides more credible EFN information because it makes use of site-specific observational data.

  • In some streams, most or all fish-accessible low-gradient reaches are situated on valley-side alluvial fans, which make them sensitive to low flows as they commonly lose some streamflow to the aquifers below (e.g., Shorts Creek). Those creeks tend to experience very low baseflows. In contrast, streams with long low-gradient valley-bottom reaches (e.g., Coldstream Creek and Mill Creek) experience substantial groundwater inflows in their lower reaches and tend to have higher baseflows than average.

  • The project has identified priority streams for streamflow restoration (where a relatively large amount of habitat could be gained by restoring heavily depleted streamflows).

  • The EFNs and CEFT values recommended herein are valid for relatively recent climate conditions. As the climate continues to change, the work will need to be reviewed on an approximately decadal scale.
  • The project has allowed the compilation of a significant volume of scientific information related to fish, fish habitat, and aquatic ecosystem needs.

  • Addressing many streams in one project has helped to efficiently leverage methodological learnings among streams.
Next steps recommended in the Phase 2 report (ONA 2020)

  • The EFN and CEFT recommendations contained in the Phase 2 report should be reviewed by key organizations, and by ONA Bands for creeks within their area of responsibility.

  • The Phase 1 report (Associated 2016) should be updated with the method adjustments and other experience gained during Phase 2.

  • The EFN and CEFT recommendations contained herein are credible and scientifically defensible. A collaborative approach should be developed for using these recommendations in water licensing and management in B.C.

  • The Phase 2 report notes that 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 (Syilx) water law development.
… for future Okanagan EFN work as recommended in the Phase 2 report

  • Collect hydrometric data. Continue operation of existing hydrometric stations in both natural and managed streams, and install additional stations as outlined in Table 4-1 of the Phase 2 report (a small portion of that table is provided in Table 4-3 of this summary report for illustrative purposes). In addition to its site-specific benefits, the additional hydrometric data will help improve hydrologic models that are used for estimating streamflows at ungauged locations for future EFN studies.

  • Refine water use estimates and measure reservoir releases. Water diversions and releases from reservoirs should be monitored and these requirements should be specified within the water licensing process.

  • Create or update operational plans for reservoirs. This will permit inclusion of EFN needs into such plans.

  • Obtain residual and maximum licensed flow estimates. Residual and maximum licensed flow datasets are not yet available for all 18 Phase 2 streams. These datasets should be completed and WUW Index percentile plots should be prepared. The impact of water use on fish habitat under residual and maximum licensed conditions can then be determined and compared among streams, which will help to identify problem areas and opportunities for streamflow restoration efforts.

  • Address over-allocation. Over-allocation in some streams is evident (through estimates of streamflows under conditions of maximum licensed water use). The licensed amounts either need to be reduced to levels that balance the needs of water users and the ecosystem, or need to be supported from off-channel storage. The increasing tendency for lower summer baseflows in recent decades revealed in the flow naturalization analysis should be considered during this exercise.

  • Develop HSI curve for Okanagan spring Chinook. An HSI curve should be developed for spring Chinook that spawn in small tributary streams. Similarly, confirmation of the Sockeye HSI curve in small tributaries would be useful.

  • Determine EFNs and critical flows for Okanagan Lake tributaries. EFNs and critical flows for all Okanagan Lake tributaries should be determined for Sockeye and Chinook spawning. Fish passage at the outlet of Okanagan Lake was implemented in fall 2019, and these species now have access to Okanagan Lake tributaries. Efforts should be focused on larger tributaries with potential to support these large-bodied species.

  • Analyze stream temperature. Stream temperature data collected during the study should be analyzed to determine whether EFNs and CEFT values warrant adjustment to mitigate the impact of high stream temperatures, recognizing that potential EFN increases are relatively limited without exceeding naturally available flows.

  • Confirm critical flows and EFNs with fish observations. Critical flows and, in some cases, EFNs should be confirmed with actual field-based fish observation data to assess the effectiveness of this approach. In particular, critical flows for juvenile fish rearing should be further investigated to confirm that the recommended critical flows are sufficient. Passage flows should be verified with fish movement information from the study streams to confirm they are appropriate.
  • Expand the climate monitoring network. Climate data in conjunction with hydrometric data will improve our understanding of how climate change affects streamflow and the ability to meet EFNs in future.

  • Restore and enhance fish habitats. Many Okanagan streams have experienced physical impacts which have reduced the quantity and quality of available fish habitat. In addition, ongoing climate change may progressively restrict the ability of the managers of Okanagan Lake dam to provide flows to the Okanagan River that fully supply anadromous fish spawning needs, which in turn could negatively impact fish populations in streams throughout the Okanagan. Accordingly, instream work to restore physical and biological functioning in areas of degraded fish habitat should be a priority throughout the Okanagan – particularly where the degradation is most severe and in areas of potentially high fisheries value. In addition to stream restoration, enhancing fish habitat to provide greater benefits than currently exist should also be considered.
… intended more generally for future EFN studies in south-central B.C.

  • Prioritize streams. Highly modified streams with high fisheries value or potential value should be prioritized for field-based EFN setting.

  • Examine reliance on naturalized flows. Uncertainty in naturalized flow estimation can be high and often habitat conditions change rapidly particularly at low flows. Thus, reliance on naturalized flows as a constraint on EFNs should be examined on a stream-by-stream basis. In the absence of recent field data, historical information on channel conditions, fish populations, and flow regimes can provide useful context for verifying naturalized flows and EFNs.

  • Determine potential for flow augmentation. Early identification of the potential for flow augmentation and resultant effects on habitat suitability assists with focusing data collection and estimation efforts.

  • Incorporate Traditional Ecological Knowledge (TEK). TEK should be incorporated into naturalized hydrograph development where available. TEK on historical ecosystem flow characteristics (predominantly wetland or side channel inundations levels) and the magnitude of the flow standards needed, as well as summer and fall low flows, could provide useful contributions and context to naturalized flow development and EFNs.

  • Observe conditions at the lowest flows. Where resources are limited, focusing WUW assessments on moderate and low flows is a reasonable trade-off because in the B.C. Interior, there is more pressure on the aquatic environment during the summer low flow period than at any other time of year – and this is when water demand is highest. Observing stream conditions during the lowest flows is necessary to properly define the lower end of the WUW curve and to determine critical flows.

  • Collect all WUW measurements in one season. Minor channel geometry changes during freshets can bias the habitat-flow relationship leading to uncertainty. However, average conditions in a given stream or reach should persist between years if representative transects are chosen.

  • Analyze water temperatures and flows. Conduct analysis of stream water temperatures and flows to guide EFN and critical flow setting.

  • Consider short-term flow fluctuations. The impacts of very short-term (i.e., days or hours) flow fluctuations within the weekly EFN time steps cannot be addressed within the EFN setting exercise, but could/should be considered in licensee-specific operating plans to make better use of water supplies (Associated 2016). This is a serious issue in some regulated streams or those experiencing very high water use.

  • Define habitats. Habitat types selected for analysis should be carefully defined to ensure consistency in transect positioning within a habitat unit.

  • Confirm that transects are representative. The number of study transects on each stream for this project was determined from stream length, variability between reaches, logistics and time constraints, and was intended to produce results representative of each stream. The degree to which this study produced representative results should be tested in one or two streams.
… to fill the knowledge gaps identified in the Phase 2 report

  • Determine flow ramping rates. The EFNs presented herein do not contain specific ramping rates. More research is recommended on ramping rates resulting from “point of diversion” withdrawals and water storage release rates.

  • Obtain fish life history information. Further information is required for Kokanee juvenile migration timing and on locally-applicable flow standards for overwintering juvenile Steelhead, Chinook and Coho, for all life stages of Sockeye, and for small-bodied Rainbow Trout adult migration.

  • Confirm actual fish population health and abundance. Fish population response to a variety of flows above and below the recommended EFNs and critical flows should be confirmed with actual fish abundance and/or health data.

  • Quantify groundwater-surface water interactions. Groundwater-surface water interactions on alluvial fans, in particular stream losses to groundwater, should be quantified to assist with naturalized flow estimation. Further, effects of channelization, groundwater pumping and urbanization on these interactions should be considered.

  • Determine channel maintenance flows. More research is needed on determining the flows required to maintain stable channel form.
… for other future EFN-related activities

  • Ensure wide distribution of the project reports, including this summary report.

  • Educate water managers and provincial permitting staff on the derivation and use of these EFNs.

  • Clarify how provincial and Syilx fisheries staff will collaborate in enhanced water allocation decision-making in light of both the WSA and the provincial UNDRIP legislation (i.e., Declaration on the Rights of Indigenous Peoples Act) adopted in November 2019.

  • Develop a method of tracking EFN decision-making and how operational flow management uses the EFNs and CEFTs provided herein.

  • Make use of the photographic record of habitat conditions at known locations and flows for educational purposes.

  • Integrate EFNs in future hydrologic modelling exercises in the Okanagan Basin.

  • Host a national EFN conference in the Okanagan with a focus on action.

  • Integrate EFNs in local government water planning.

  • Seek collaborative funding from all levels of government for fish habitat restoration and enhancement


Appendix A

EFN Phase 1 Report Collaborative Development of Methods to Set Environmental Flow Needs in Okanagan Streams

Appendix B

Streamflow Dataset Development Methods Report (29Mb PDF) Recommended Methods for the Development of Streamflow Datasets to Support the Application of the Okanagan Tennant Method in Okanagan Streams

Appendix C

Streamflow Dataset Report (30Mb PDF)- Streamflow Datasets to Support the Application of the Okanagan Tennant Methods in Priority Okanagan Streams

Appendix D

EFN Phase 2 Report Environmental Flow Needs Assessment in the Okanagan Basin