Building upon modeling efforts in two river systems, the International Osoyoos Lake Board of Control has a complete hydrologic computer model covering the entire Osoyoos Lake basin.
This model, as detailed in the recently published final project report, combines three recently completed computer models covering Osoyoos Lake, the upstream Okanagan River basin and the Similkameen River basin, which meets downstream of Osoyoos Lake at the Okanogan River. Both the Okanagan and Similkameen affect the water levels on Osoyoos Lake, and understanding how water moves through those systems under different conditions is vital to understanding how Osoyoos Lake responds, according to Martin Suchy, Canadian secretary to the Osoyoos board and a hydrologist with Environment and Climate Change Canada. Osoyoos Lake provides critical habitat, water storage, and recreational opportunities, and the International Joint Commission (IJC) Order seeks to balance these needs for this transboundary water body.
This model, in turn, is valuable for the board as it works to follow the IJC’s operating Order. The Order requires water levels to be maintained within specific ranges throughout the year, a range known as the rule curve. In 2013, the IJC issued a Supplemental Order providing updated guidance based on studies conducted in the 2000s and feedback from water users and area residents. Suchy said this Supplemental Order allows for adaptive management of Osoyoos Lake levels by distinguishing between normal and drought years, and by providing greater operational flexibility when drought conditions are met. In recent years, the board has seen frequent drought conditions where the freshet volumes from both rivers are lower than average.
“In 2019 when we started talking about developing these models and the need for them, it was starting to become apparent that there wasn’t really a ‘normal condition year’ any more,” said Suchy. “It was very much extremes. We had flood and high lake level years and a number of dry years, or drought condition years.”
Scott VanderKooi, US Co-Chair to the Osoyoos board and Center Director of the U.S. Geological Survey Washington Water Science Center, said this modeling effort builds upon work completed by the Canadian Okanagan Basin Water Board (OBWB), a domestic organization that promotes collaborative management of the Okanagan River basin. The OBWB had contracted a flood mapping modeling project to Northwest Hydraulic Consultants (NHC), which generated a hydrological model of the river system geared toward flood levels and risks, according to VanderKooi.
“When [The OBWB] were almost done, the Osoyoos board decided to develop a hydrologic model of the Similkameen basin,” VanderKooi said. “There was limited understanding of how water availability from the Similkameen might change in the future, because it can flow an order of magnitude higher during a freshet than the Okanagan, but also frequently experiences dry conditions.”
The Okanagan River work concluded around 2020, and the Similkameen effort got under way in 2021 through NHC. VanderKooi said the OBWB had done a lot of elevation mapping in Canada and that the Osoyoos board was able to extend the mapping into the US portion of the system using International Watersheds Initiative (IWI) funding through the IJC. The IWI has a framework that provides funding for projects that help IJC boards investigate how a shifting climate could impact their responsibilities and duties. Accurate, high-resolution elevation maps form the backbone of hydrologic models by showing where water is likely to flow and how much storage capacity lakes and reservoirs hold.
Once the Okanogan and Similkameen basins each had an individual hydrologic model, VanderKooi said the board then contracted NHC to modify the Okanagan model to low flow conditions and weave them together. The confluence of the Similkameen and Okanogan Rivers is just south of Osoyoos Lake; during high flow events this confluence can cause backflow up into Osoyoos Lake and raise water levels. The two river systems are connected, and both are critical for providing water to downstream users.
The result is an integrated model capable of running climate projections through its simulations of the Similkameen/Okanagan/Osoyoos systems. Furthermore, it includes an “operational model” that takes into account the operation of Zosel Dam at the downstream end of Osoyoos Lake and additional dam systems upstream at Okanagan Lake. This can provide a full view of how different conditions and operational decisions will impact water levels throughout the system.
According to the final report, local climate data projects that the Okanagan and Similkameen basins will experience smaller spring freshets, lower summer flows, and more rainfall in winter under scenarios of increasing temperatures in the future.
Given these projected changes, the data available suggests that Osoyoos Lake is expected to experience lower water levels due to smaller spring freshets, and may wind up being more prone to drought conditions.
The model results highlight the importance of continued efforts to understand and manage waters shared between countries and basins now and into the future. We can’t control what weather conditions reach Osoyoos Lake, but better information now will ensure the board will be better prepared for extreme circumstances.
