Maintaining healthy and productive aquatic ecosystems in and around streams, lakes, and rivers requires careful management of human impact on aquatic ecosystems, including limiting damage to streams and riparian zones, and withdrawal for drinking water or agriculture.  

The Okanagan population of about 350,000 is increasing at a faster rate than both the BC and Canada-wide averages, increasing demands on water supply.  Simultaneously, climate change is reducing water supply and increasing water demand.  The annual hydrologic cycle is shifting towards a smaller and earlier spring snowmelt peak flow, and summers are becoming longer, hotter, and drier. These changes are putting aquatic ecosystems under ever-increasing stress, particularly in late-summer and early-fall, a critical period in the life cycle of several species of Okanagan fish.

There are many factors that control the health of aquatic populations in a stream.  We typically simplify these factors into a single measure known as the Environmental Flow Need (EFN). The EFN is a time-varying rate of streamflow that provides sufficient water to enable the aquatic ecosystem to be healthy and sustainable.  It is not so low that the ecosystem experiences undue stress, nor so high that water remains in the stream over and above the amount required to enable the ecosystem to thrive. Although a specified flow rate is a simplified representation of the factors that influence ecosystem success, flow rate is a convenient measure because humans can directly influence streamflow, and thereby aquatic health.

The British Columbia government is responsible for allocating water for human use.  For 109 years, water allocation was governed by the BC Water Act.  However, on February 29, 2016, the BC Water Act was replaced by the Water Sustainability Act.  The new Act treats groundwater the same way as surface water.  The Act requires the province to consider the Environmental Flow Need of a stream before allocating water from a stream or from a nearby groundwater aquifer connected to the stream.  This key new provision was included in the Act, partly in recognition of the existing and growing pressures on Okanagan aquatic ecosystems.

There are several methods available for determining the EFNs of streams, subdivided into office-based methods and field-based methods.  Over the past 35 years, several attempts have been made to determine EFNs in Okanagan streams.  However, most previous attempts have relied on office-based methods, which have not been accepted as sufficiently credible to support water allocation decisions because they have not considered the stream-specific factors that affect aquatic ecosystem health in individual streams.

Recognizing the increasing pressures on water supply, combined with the anticipated requirements of the Water Sustainability Act, in 2015 the Okanagan Basin Water Board joined forces with the Okanagan Nation Alliance, and the Province of BC to determine the Environmental Flow Needs of Okanagan streams.  Phase One of the project recommended an Okanagan-appropriate desktop method and an Okanagan-appropriate field-based method of determining EFNs.  The desktop method is used in low risk circumstances, while the costlier, more intensive field-based method is intended for higher risk situations, such as to support water allocation decision-making. 

Associated Environmental led a broad-based consulting team to complete Phase One.  The team recommended a modified form of the Tennant method (Okanagan Tennant method) for a first-cut EFN estimate, and a modified form of the Weighted Useable Width method (Okanagan WUW method) for developing a more comprehensive field-based EFN estimate.  Both the original Tennant method and the original Weighted Useable Width method have been successfully applied in the Okanagan previously.  The Phase One report lays out a sequence of steps for implementing each of the two Okanagan-specific methods. However, neither method is overly prescriptive, offering flexibility in the level of effort at each step, and experience and good judgment are required. The Phase One technical team collaborated with many agencies and stakeholders to develop recommendations, which were accepted and implemented.  

The Okanagan Nation Alliance-led Phase Two includes developing Okanagan Tennant–based EFNs on 19 streams, followed by Okanagan Weighted Usable Width–based EFN estimates on 11 of these 19 streams.  Associated Environmental continues to play both technical and senior advisory roles in the ongoing project.  

The Phase One report is a living document that will be updated, along with the EFN methods, as new knowledge becomes available. The document will be translated into the Okanagan language so Okanagan elders and knowledge-keepers can more fully contribute important historical context. 

Developing credible, scientifically-based EFNs on Okanagan streams will allow provincial water allocation officers to comply with the new Water Sustainability Act, better protect aquatic ecosystems, raise the level of awareness of the value of aquatic ecosystems among many stakeholders, and provide greater certainty about the level of development that can take place while still maintaining healthy, sustainable aquatic ecosystems.