ESSA Technologies Ltd. and Solander Ecological Research. (2009). Phase 2: Okanagan Water Supply and Demand Project: Instream Flow Needs Analysis. Kelowna: Prepared for the Okanagan Basin Water Board.
The Okanagan River and tributary streams throughout the basin provide critical spawning and rearing habitat for salmon, trout and other resident fish species. As a result of increasing water demands, dams for flood control and continuing land development, fish habitat in many subbasin watersheds has been impacted or even eliminated. The Okanagan Water Supply and Demand Project was initiated in response to increasing concern over the remaining amount of surplus water available for continued growth and the impacts of climate change on water supplies and environmental water needs. A major task within our instream flow needs (IFN) project was to use modeled information on “naturalized flows” to determine IFN for fish and other aquatic biota at specific areas (called “nodes”) that have been defined for the Okanagan Water Supply and Demand Project. These nodes consist of key tributary mouths, mainstem lakes, and key locations along the Okanagan River. For the first time, this report documents a comprehensive approach for defining basin-wide default instream flow needs where site specific data on biophysical linkages does not exist. The approach uses a combination of two peer reviewed IFN methodologies for standard setting that are accepted in the scientific literature and supported by provincial and federal government biologists. Where available, these “default” guidelines are supplemented by instream flow recommendations that currently exist following site-specific IFN studies or water use planning agreements.
Our first IFN method was the Hatfield and Bruce (2000) meta-analysis approach, based on over 1500 habitat vs. flow curves from 127 physical habitat simulation (PHABSIM) studies throughout western North America. We used this method to generate regression-based predictions of optimal flows for spawning and rearing of kokanee, sockeye, rainbow trout, steelhead, Chinook and coho salmon. While this approach helps to define optimal flows for rearing/spawning salmonids, it does not assess required flows for other fish species, other biota, wetland linkages, channel evolution, etc. Therefore we incorporated BCIFN Phase II instream flow guidelines to generate minimum flows required for broader ecosystem needs (Hatfield et al. 2003). The BCIFN method estimates minimum flow thresholds throughout the year to maintain the key features of a particular stream’s natural hydrograph and minimize risk to fish and other aquatic biota. The BCIFN approach also caps recommended extraction/allocation of water from a stream at a defined threshold (a maximum diversion rate). We subtracted this recommended maximum diversion rate from the weekly naturalized flows to calculate a recommended watershed conservation flow. The intent of watershed conservation flows is to ensure sufficient water remains in the streams during the high flow months to fulfill geomorphic needs and promote broader ecological functions. Watershed conservation flows are required in roughly one in five to one in ten years.
The instream flow recommendations in this report are largely based on the needs of fish, especially “sentinel” indicator species (e.g., kokanee, sockeye, rainbow trout). Thus, the instream flows recommended do not represent a true ecosystem assessment for all types of aquatic and riparian organisms. Such an undertaking would require a much larger project, one that included considerable time and resources for field assessments and monitoring. We have however made an effort to consider how our default guidelines do or do not support the requirements for a number of other aquatic-dependent organisms which are federally or provincially listed as species of concern.
Another important feature of this report is alternative IFN “scenarios”, which represent different mixes of IFN method, focal species/life-stage and risk tolerances. For example, Chinook and coho salmon IFN needs were also evaluated even though they are not currently present in Okanagan tributary streams. These may be useful in the evaluation and design of future restoration initiatives. The IFN scenario results for Okanagan nodes are presented in a variety of formats – as weekly IFN values for nodes uploaded to the OKWaterDB, as graphical representations of weekly IFN recommendations (minimal and optimal flows) at individual nodes, as tables of exceedance probability matrices (linked to “traffic light” plots in some cases) across all tributary nodes, and as hazard maps of inherent hydrologic risk for a subset of IFN scenarios.
It is very important to recognize that consideration of socio-economic water demands and the appropriate ecological trade-offs were outside the scope of this study. Anthropogenic water needs are typically readily available from regulatory authorities (e.g., water license information) and at the forefront of legal agreements (e.g., flood protection, recreational flow needs). Where we were able to acquire water use agreement information related directly to fish flow needs, as in the case of Okanagan River, Trout Creek and Mission Creek, we compared these instream flow targets to the default guidelines generated by our BCIFN and meta-analysis methods.
Here is a synopsis of key findings:
- Naturalized flows at individual nodes varied in their ability to achieve optimal fish flows for different salmonid species during critical life-stage periods. In general, however, naturalized flows were sufficient in most years to achieve mean optimal flows for rainbow trout and steelhead spawning but often failed to provide optimal flows for kokanee and sockeye spawning in tributaries in which they occur, or for (hypothetical) spawning of Chinook and coho in selected key tributaries. Achievement of optimal rearing flows for rainbow trout, steelhead and coho varied throughout their year-long residence in the streams but optimal flows were generally achieved within the shorter time period of Chinook rearing.
- Tributary nodes in the northeast section of the Basin displayed a better inherent ability to achieve IFN flows defined for different species and during different time periods of the year, whereas the opposite seemed to be the case for tributary nodes in the northwest section of the Basin (i.e., poorer ability to meet IFNs).
- These results reflect the fact that the Okanagan is a naturally dry region, and even in the absence of human water use (as illustrated by naturalized flows), flows are frequently sub-optimal for fish production when considering the flow levels these species prefer throughout their range.
We compared exceedance probabilities for select regulated flows and naturalized flows (this comparison was not possible for most nodes as we did not have the net water availability time series that are to be generated by the water balance modelling project). This comparison indicated that at some nodes recommended BCIFN minimum risk flows were achieved more frequently in the late summer dry period with regulated flows than with naturalized flows. This was presumably due to increased storage in some watersheds during the freshet with subsequent release of this stored water later in the summer. However, regulated flows generally met BCIFN minimum risk flow thresholds less often than naturalized flows during other critical time periods (e.g., mid-winter). Increased water storage (where possible) linked to ecological releases targeting critical fish needs represents a valid management avenue for better achievement of instream flows needs – especially in the context of projected future climate change. Developing such strategies (e.g., ecological water reserves) requires an acceptance that instream values have a right comparable to anthropocentric rights (though the relative weights of these rights will vary case by case, and with human values). Historically water has often been allocated among priority rights holders first with instream needs being allocated as an afterthought or only if “excess” water exists. Hopefully this value-system will evolve with future implementation of the Okanagan Sustainable Water Strategy, which emphasizes attaining a better balance between human and ecosystem needs.