Ice jams occur regularly at the southern outlet of Osoyoos Lake, which spans the border between the State of Washington and British Columbia, Canada. In recent winters, ice jams caused (1) decreases in downstream discharge that may adversely affect salmon spawning habitat and (2) short-duration lake-level rise that can interfere with lake level management agreements. In response, water managers sought to understand the environmental conditions associated with the historical ice-jam occurrences on Osoyoos Lake. Researchers compiled datasets of discharge, lake level, and air temperature from four meteorological and three hydrologic stations near Oroville, Washington, to determine “ice-jam” or “non-ice-jam” days from 1942 to 2024.

After confirming known ice jams since 1994 using Landsat 8–9 and Sentinel–2 satellite imagery along with discharge, lake level, and air temperature data, researchers designated ice-jam days. They conducted statistical analyses to examine environmental conditions associated with ice-jam occurrences on Osoyoos Lake. Statistical tests indicated significant differences in wind speed, wind direction, and air temperature between ice-jam and non-ice-jam days. A linear discriminant-analysis model correctly predicted 12 of 13 historical ice-jam days since 1994 and determined that ice jams are more likely under westerly and northwesterly winds near or above 10 kilometers per hour (km/h) and minimum temperatures near or below –9.4 degrees Celsius (°C). An analysis of historical discharge suggests that ice jams have occurred since at least the 1940s, but 13 ice jam days occurred in the past decade (2014–2024), exceeding any previous decade. The daily minimum air temperature in the Osoyoos Lake region has increased at a rate of 0.021 °C per year since the 1940s, but ice jams usually occur in winters with colder average temperatures.

Ice jams are accumulations of ice that partially block water from flowing downstream in rivers and lakes. Ice jams form at the shallow outlet of Osoyoos Lake, which drains into the Okanogan River at the border of the United States and Canada. These ice jams can temporarily reduce river flow downstream, which can harm salmon habitat and cause short lived increases in lake levels that complicate international agreements for managing water levels.

To better understand when and why these ice jams form, researchers from the U.S. Geological Survey examined historical records of river flow, lake level, and air temperature data from stations near Oroville, Washington (located just south of the lake outlet), for the years 1942–2024. Researchers used satellite images and environmental data during 1994–2024 to confirm known ice jams and then identified “ice jam days” for that period.

The team compared weather conditions on ice jam days and non-ice-jam days. They found that ice jams are more likely to form when winds blow from the west or northwest at speeds of about 10 kilometers per hour or more, and when minimum temperatures drop to –9.4 degrees Celsius or lower. A statistical model based on air temperature, wind speed, and wind direction correctly identified nearly all known ice jam days since 1994. While the statistical model identified some days without ice jams as ice-jam days, no ice-jam days occurred outside of the range of wind and temperature conditions identified.

Although ice jams have occurred since at least the 1940s, they have become more frequent in recent years: 13 ice jam days occurred during 2014–2024, more than in any previous decade. Even though winter temperatures in the Osoyoos Lake region have risen slightly over time, ice jams tend to occur during colder than average winters.

Understanding the conditions that lead to ice jams can help decision-makers better anticipate when ice jams may occur and plan for their potential effects on salmon habitat and lake level management.