The 2012 Great Lakes Water Quality Agreement states that “the Waters of the Great Lakes should be free from other substances, materials or conditions that may negatively impact the chemical, physical or biological integrity of the Waters of the Great Lakes.”
Assessment highlights
Measures of watershed impacts, including Land Cover, Hardened Shorelines, and Water Quality in Tributaries, which can affect water quality, are assessed as Fair with No Trend over the reported 10-year period. Status assessments are not prepared for the Changes in Physical Conditions sub-indicators; however, long-term trends are presented.
Watershed Impacts
All lake basins experienced an overall increase in population over the most recent 10-year period for which census data are available (2010-2020 for the U.S. and 2011-2021 for Canada), with the greatest rate of population growth occurring in the Lake Ontario basin. It is expected that the population within the Lake Ontario basin will continue to increase rapidly, particularly in the Greater Toronto Area. Rapid population growth often leads to urban development and loss of natural land cover.
Based on 2020 data, the overall Great Lakes basin land cover is classified as approximately 8% developed, 26% agriculture, and 65% natural. However, there is a high degree of variability in the percentage of land cover type between the lake basins. For example, the Lake Superior basin has a large amount of natural lands (97%), compared to the Lake Erie basin (21%). There is a positive relationship between the extent of natural lands and water quality. Runoff from agricultural and developed lands may contribute to degraded water quality and ecological conditions. Riparian zones (i.e., land along the banks of a river or stream) throughout the Great Lakes basin are generally dominated by natural lands (75% overall), much of which is forested (53% overall). However, this varies greatly between lake basins. Forested riparian zones provide essential ecosystem services including decreasing runoff and erosion and regulating water temperatures.
From 2009 to 2020, the amount of shoreline that has been hardened (any placement of material used to armor the shoreline to offer protection from waves and water level changes) increased for the Great Lakes overall. Shoreline hardening can alter habitats and sediment transport, while reducing ecosystem services. Currently, almost one quarter of the assessed Great Lakes shoreline is either moderately or highly hardened.
Tributaries play an important role in transporting surface water to the lakes. Watershed land use and stressors can directly impact water quality in receiving tributaries. Water quality data from 72 Canadian Great Lakes tributaries were evaluated using an index based on concentrations of ammonia, chloride, copper, iron, nitrate, nitrite, phosphorus and zinc. Index scores support an overall Fair assessment for the Canadian Great Lakes tributaries. Additionally, long-term trends (1970 to 2019) generally indicate increasing chloride concentrations, no trend in nitrate concentrations, and decreasing phosphorus concentrations. Index scores confirm that overall tributary water quality is influenced by land use, with more urbanized or agricultural watersheds typically having poorer scores.
Long-term Trends in Great Lakes Physical Conditions
Long-term datasets show increases in annual precipitation amounts in most of the Great Lakes basin, increases in summer surface water temperatures in all five lakes, and a reduction in overall Great Lakes annual maximum ice cover. Lake Superior experienced a 34% decrease in maximum ice cover from 1973 to 2023, which is the largest decline in the Great Lakes. Surface water temperatures in lakes Superior, Michigan and Huron have increased at a rate of 0.5 °C per decade since 1980; Lake Erie surface water temperatures have increased 0.2 °C per decade since 1980. In Lake Ontario, surface water temperatures over the past decade are approximately 1 °C higher than the average from the 1960s to the early 1990s. Summer surface water temperatures are driven by both air temperatures and the previous winter ice cover extent. At a 100-year time scale, average lake levels in lakes Superior, Michigan and Huron have not significantly changed, and average lake levels increased in lakes Erie and Ontario. After experiencing historic high lake levels in 2017–2020, all lakes have returned to levels similar to the average from the 1960s to the early 1990s. Short-term trends for the physical conditions sub-indicators vary annually, leading to a No Trend assessment over shorter periods.
Changing water temperatures, ice cover and lake levels can affect Great Lakes habitat quality, including fish spawning areas, as well as other important fish habitats. For example, some fish species rely on the extent and/or duration of ice cover to protect their eggs from turbulence caused by wave action. These physical changes can also contribute to changes in native species range distributions and create conditions that favor the establishment of non-native species.
Forests, wetlands and other natural land cover that help maintain water quality are more prevalent in the northern regions of the Great Lakes basin
Annual maximum Great Lakes ice coverage has decreased since 1973
Mean summer surface water temperature in Lake Superior, represented as deviation from the long-term mean, has increased in recent decades
*The anomaly approach compares recent trends to a defined baseline to highlight changes over time. This approach helps put historical data in context with recent ecosystem changes.
Sub-indicators supporting the Watershed Impacts assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Land Cover | Good & Unchanging | Fair & Undetermined | Fair & Undetermined | Poor & Undetermined | Fair & Undetermined |
Hardened Shorelines | Good & No Trend | Good & Deteriorating | Good & Deteriorating | Poor & Deteriorating | Poor & Deteriorating |
Water Quality in Tributaries | Undetermined & Undetermined | Not Assessed | Fair & No Trend | Poor & No Trend | Fair & No Trend |
Human Population | Increasing | Increasing | Increasing | Increasing | Increasing |
Sub-indicators supporting the Changes in Physical Conditions assessment
Sub-Indicator | Lake Superior | Lake Michigan | Lake Huron | Lake Erie | Lake Ontario |
Precipitation Amounts (1950-2023) | No Trend | Increasing | Increasing | Increasing | |
Water Levels (1918-2023) | No Trend | No Trend | Increasing | Increasing | |
Surface Water Temperature (1980-2023) | Increasing | Increasing | Increasing | Increasing | Increasing |
Ice Cover (1973-2023) | Decreasing | Decreasing | Decreasing | Decreasing | Decreasing |
Status
