Environmental flow needs assessment for salmonids in the Coldwater River

Introduction

Anthropogenic climate change is rapidly shifting the hydrology of many watersheds across British Columbia. Meteorological droughts, defined as a precipitation deficit over a period of time, are becoming more common globally as the world warms (IPCC 2022). In British Columbia, droughts are already impacting ecosystems, communities, and the economy. Summer rainfall and winter snowfall are declining, with the worst impacts expected in coastal and southern interior regions (CCI 2019). Precipitation deficits often lead to impacts on streamflow and groundwater recharge, with consequences for aquatic ecosystems.

Climate change can exacerbate hydrological droughts through direct and indirect pathways. Direct pathways reduce the volume of water entering a system through changes in summer precipitation (Saha 2015), reductions in winter snowfall and snowpack, earlier spring melts, and increases in summer and winter air temperatures (Dierauer 2021). Indirect pathways disturb natural hydrologic processes which influence streamflow. For example, the loss of native forests to wildfire and insect infestations–both of which are well-documented symptoms of climate change (Caroll et al. 2003; Kirchmeier-Young 2018)–can alter rates of groundwater recharge (Allen 2010), impacting streams that rely on groundwater inputs during the dry months. Droughts can also increase anthropogenic water demands to meet agricultural, industrial, and municipal needs, further exacerbating the impacts on aquatic ecosystems (Dierauer 2018).

Droughts have severe and lasting impacts on Pacific salmon populations, particularly for species that rear in freshwater such as coho, stream-type Chinook, and steelhead. Droughts reduce useable habitat for salmonids (Deitch et al. 2018), reduce the window for ocean-going migration (Kastl et al. 2022), affect population productivity (Warkentin et al. 2022), and result in poor water quality including high water temperature (Martins et al. 2010) and low dissolved oxygen (Ficklin et al. 2013). Pacific salmon populations in British Columbia have already experienced significant declines from pre-colonial abundance due to cumulative effects of overfishing, habitat loss and degradation, and climate change. In the freshwater environment, droughts are a significant barrier to the recovery of already-at-risk populations that spawn in the semi-arid interior regions of British Columbia.

Environmental Flow Needs (EFNs) is a concept in hydrology that describes the timing and volume of flow required for the proper functioning of a stream ecosystem. Stream features such as riffles, pools, and glides require a specific range of flow to perform their respective ecosystem functions (e.g., fish rearing, benthic macroinvertebrate production, stream oxygenation, fish spawning, cold water refugia) (Acreman 2016). As such, when stream flows are reduced, these functions can be impacted. In British Columbia, EFNs are defined in the provincial Water Sustainability Act (WSA) using a Critical Environmental Flow Threshold (CEFT). The CEFT is defined as the flow below which “significant or irreversible harm to the aquatic ecosystem of the stream is likely to occur” (WLRS, 2024). Under Section 88 of the WSA, the decision-maker can use the CEFT of a stream, in addition to other biological and environmental information, to make an order to regulate diversion of water in a stream or from aquifers that are determined to be hydraulically connected to the stream. Understanding the EFN and the CEFT of a stream is therefore critical to inform water management decisions, particularly in salmon-bearing watersheds with a high degree of anthropogenic water demands (Pahl-Wost, 2013).

The goal of this study was to model the relationship between flow and freshwater habitat for anadromous salmon species in the Coldwater River using the Okanagan Weighted Usable Width (WUW) method (ONA 2020). This relationship is the first step in determining EFNs, a process that is often a collaborative effort between multiple levels of government, Indigenous government, scientists, and water users. Since 1982, provincial water managers have used a Fisheries Resource Maintenance Flow (FRMF) value determined by Kosakoski & Hamilton (1982) to inform water allocation decisions and drought response (Nicola Water Use Management Plan 2003). In the nearly half-century since, the Coldwater River has experienced both significant shifts in climate and precipitation regimes, and has undergone changes to stream morphology following the 2021 floods. EFN science has also evolved to include aspects of Traditional Ecological Knowledge and acknowledge differences in flow requirements at the species, life stage, and ecosystem levels. These factors highlight the need to reassess EFNs in the Coldwater River, particularly in the context of climate change.

We also analyzed long-term streamflow data of the Coldwater River to assess trends in summer flows, particularly as they relate to the CEFT. By doing so, we aim to present target flows that water managers should strive to meet and highlight the urgency of which action is needed.

Methods

Study site

The Coldwater River is a major tributary of the Nicola River in BC’s southern interior. The Coldwater is part of the broader Thompson and Fraser watersheds and is situated in nłeʔképmx and Syilx territory and home to five band governments: sp’ax̌ʔmi and nɬq’aɬməlʔx (Upper Nicola), sulús (Lower Nicola), nc̓ə́ɬetkʷu (Coldwater), sxéxn̓x (Shackan), and nwéyc (Nooaitch). The river has a snow dominated hydrograph characterized by peak flows during the spring freshet, typically in May or June, with flows diminishing steadily throughout the summer. The Coldwater River also has rain dominated features akin to coastal systems in which there is often another marked increase in streamflow with Fall rain events – which are becoming more common with climate change (BGC 2022).

The Coldwater is home to three populations of Pacific salmon identified as at-risk or threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC): Interior Fraser coho (COSEWIC 2016), Interior Fraser Steelhead (COSEWIC 2020), and spring-run Lower Thompson stream-type Chinook (COSEWIC 2020). These populations have declined so precipitously that members of the Coldwater Indian Band, to whom salmon are an important traditional food, have not been able to fish for coho salmon since the 1980’s and steelhead since the early 2000’s (Coldwater Indian Band 2018). 

Anthropogenic water demands in the Coldwater are high due to a robust ranching sector (Nicola Water Use Management Plan 2003). Excluding water licences for conservation and storage purposes, the Coldwater River watershed currently has 45 active water licences that hold a cumulative authorized withdrawal of 5,634,367 m³ per year. Roughly half of the licenced volume is for agricultural irrigation purposes, with the remainder for municipal purposes. Since the 1950’s, the region has experienced a general trend towards intensifying and lengthening low flow periods in the summer. In November 2021, the Coldwater River experienced a 1-in-200 year flood event which dramatically altered stream morphology and caused widespread bank instability, significant loss in riparian vegetation, and a marked increase in the proportion of armored banks (Davidson et al. 2024 & BGC 2022).

Historical streamflow data

We used historical hydrometric data collected by the Water Survey of Canada and compiled in the HYDAT database. Data from both Water Survey of Canada hydrometric stations “Coldwater River at Merritt” (08LG010) and “Coldwater River at Brookemere” (08LG048) were analyzed. The Merritt station is downstream of the majority of surface water withdrawals while the Brookemere station is upstream of these withdrawals. Data were imported into R version 4.3.2 using the packages tidyhydat and dplyr, with figures generated using ggplot2. Streamflow data for 2024-present was not available at the time of analysis and thus was not included in the analysis. 

Tennant EFNs

Prior to fieldwork, we referred to literature and engaged with local decision-makers to determine existing flow values that were being used for management. Kosakoski and Hamilton (1982) estimated a Fisheries Resource Maintenance Flow (FRMF) for the Coldwater to be 1.42 m³/s at Merritt. They also estimated the CEFT of the Coldwater River to be 0.84 m³/s at Merritt. These two values are most referenced by decision-makers to guide water management.

We used the Okanagan Tennant method, an adaptation of the Tennant Method (Tennant, 1976) developed by the Okanagan Nation Alliance in 2020 (ONA 2020), to determine desk-based EFNs. This method accounts for natural flow ranges and variability in assessed streams. 

Naturalized flow values for the Coldwater were determined by accounting for withdrawals and inputs into the stream then adding or subtracting these values to residual flow data measured at a hydrometric station. As the Coldwater River lacks controlled water storage, monthly stream withdrawal data was sufficient for flow naturalization. This data was provided and verified by members of the Ministry of Water, Lands and Resource Stewardship (WLRS pers. comm). Withdrawal data was added to the median of mean monthly discharge (MMMD) for the entire period of record from the Coldwater River at Merritt hydrometric station. This allowed us to account for monthly withdrawals and calculate the naturalized median of the mean monthly discharges (NMMMD). NMMMD values are useful for understanding how often flow standards are being met and assess when flow is potentially a natural limiting factor. LTMAD was calculated by summing the average annual withdrawal and the mean annual discharge (MAD) of 8.13 m³/s, which resulted in an LTMAD value of 8.32 m³/s.

Fish periodicity was provided by the Fisheries and Oceans Canada Stock Assessment Division. Steelhead migration and spawning information was verified in the available literature (Renn, et al. 2001). Flow standards, presented as percentages of naturalized long-term mean annual discharge (LTMAD), were applied to residual flow data to arrive at Tennant EFNs (TEFN). Critical Environmental Flow Thresholds (CEFT) were also calculated as percentages of naturalized LTMAD (ONA 2020).

Fish periodicity was applied to flow standards to determine Tennant EFNs for each month according to the species and life stage present. NMMMD was subtracted from flow standard EFNs to identify environmental flow surpluses and deficits. 

Site selection

We selected nine sites throughout the Coldwater River from its headwaters along Upper Coldwater Road (49.648351, -121.029190) to its confluence with the Nicola River at Merritt (50.110354, -120.803555). Reaches were relatively evenly distributed throughout the river, and were selected based on access and perceived salmon habitat value considering depth and flow (Appendix 1).

Reach selection was informed by existing habitat mapping data and knowledge of planned or ongoing habitat restoration works provided by Scw’exmx Tribal Council. All reaches were relatively straight sections of stream with minimal braiding to ensure consistent channel conditions throughout the course of the study. Each reach contained two transects, one riffle and one glide, to assess both spawning and juvenile rearing habitat, respectively. Riffles were defined as shallow sections of stream with visible surface turbulence, with a wetted width to average depth ratio > 50. Glides were defined as shallow sections of stream with little to no visible surface turbulence, with an intermediate wetted width to average depth ratio of 21-49. Transects were selected during the first site visit in early July when the stream was near the long term mean annual discharge (LTMAD). As stream flows dropped during the study period, some transects that were first designated as a glide exhibited riffle-like characteristics during extreme low flow periods in late summer. Retroactive analysis of the summer 2024 hydrograph indicated that the low flow period was typical of recent conditions. Transects were spaced no more than 50 river meters apart.