Watershed Impacts and Climate Trends

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.”

Status: Fair
Trend: Unchanging

Assessment highlights

Land-based stressors which can affect water quality are assessed as Fair with an Unchanging trend. Status is not currently assessed for the Climate Trend sub-indicators; however long-term trends are presented.

Watershed Impacts

The northern portion of the Great Lakes basin remains largely undeveloped and dominated by natural cover. The southern portion is more populated with less natural cover. Development, agriculture and road density are stressors on the Great Lakes ecosystem, especially in areas with larger populations. Urban and agricultural lands are important to the Great Lakes region because they help support people and the economy, however, the water quality in these areas is at higher risk of impairment.

Most of the lake basins experienced an overall increase in population over the current 10-year period that census data are available (2010-2020 for U.S. and 2011-2021 for Canada), with the greatest population growth occurring in the Lake Ontario basin, especially in Canada. In fact, it is expected that the Lake Ontario basin’s population will continue to increase rapidly, particularly in the Greater Toronto Area, further impacting the basin.

Based on 2015 data, overall Great Lakes basin land cover is classified as approximately 66% natural land cover. There is a high degree of variability in the percentage of land cover type between the lake basins. For example, Lake Superior has a high amount of natural land cover (97%), compared to the Lake Erie basin (21%). From 2000 to 2015, there was an estimated net increase in developed land of 2,893 km2. Various other land cover types changed in overall extent over this period, including an estimated net decrease in forest land of 2,900 km2 and in wetlands of 583 km2.

Research has shown that Great Lakes water quality benefits from forest cover within a riparian zone (i.e., land along a lake, river or stream). Forested riparian zones provide essential ecosystem services such as decreasing runoff and erosion and regulating water temperatures.

The amount of Great Lakes shoreline that has been hardened (any placement of material used to armor the shoreline to offer protection from waves and water level changes) has increased. Shoreline hardening can cause changes in habitats and sediment transport. 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, however, watershed land use and stressors directly impact water quality in receiving tributaries. Water quality data from 72 Canadian Great Lakes tributaries were evaluated using an index based on ammonia, chloride, copper, iron, nitrate, nitrite, phosphorus and zinc. The results of the index scores confirm that overall tributary water quality is influenced by land use with poorer scores typically being seen in more urbanized or agricultural watersheds.

Historical Climate Trends

Long-term climate data generally show basin-wide increases in precipitation, increases in summer surface water temperatures and a reduction in Great Lakes ice cover. Studies have shown that Lake Superior is one of the fastest warming large lakes in the world. Lake Superior also has the greatest long-term decline in ice cover in the Great Lakes, experiencing a 35% decrease in maximum ice cover from 1973 to 2020.

In general, water levels in lakes Superior, Michigan, Huron and Ontario show no significant overall average change over the last 100 years, while Lake Erie has seen increasing water levels. However, short-term trends are quite variable. For example, water levels in all the Great Lakes have increased over the past 10 years and Lake Ontario experienced its highest monthly mean levels in over 100 years during June of 2019. Due to the many hydrological influences on lake levels, it is difficult to determine with certainty if these water level trends are within natural climatic variability or are longer-term trends that will continue in the future. However, the recent increasing water levels align with the very high precipitation amounts experienced over the most recent ten-year period. In fact, the total precipitation amounts measured from 2011 to 2020 for the Great Lakes basin were higher than any other ten-year period since 1950.

Shifts in climate can affect Great Lakes habitats, including impacting spawning areas and other habitats for fish species, the extent and quality of coastal wetlands and forest composition. Shifts in climate can also alter biological communities, such as contributing to the northward migration of native and invasive species and creating conditions that favor some invaders over native species. Great Lakes water quality can also be impacted by increases in runoff, changes to contaminant and nutrient cycling and increases in algal blooms.

Map of the land cover classifications throughout the Great Lakes Basin. Southern parts of the basin contain higher density of yellow coloration, indicating more agricultural land cover. Northern regions of the basin tend to contain higher amounts of green coloration, indicating more natural land cover. Metropolitan areas such as Detroit and Toronto are red, indicating developed areas.

 

Natural land cover is more prominent in the northern regions of the Great Lakes basin

 

Water levels increased on all lakes over the 2010-2020 period

Bar plot of total annual precipitation anomaly (in terms of percent compared to the 1961-1990 mean) in the Great Lakes Basin from 1950-2020. Blue bars indicate years with precipitation below the 1961-1990 average; orange bars indicate years with precipitation amounts above the 1961-1990 average. A line showing the full-term trend indicates that precipitation is increasing since 1950.

 

— long-term trend

 

Great Lakes basin total annual precipitation has increased since 1950

Sub-indicators supporting the Watershed Impacts assessment

Sub-Indicator

Lake Superior

Lake Michigan

Lake Huron

Lake Erie

Lake Ontario

Good and Improving

Fair and Unchanging

Fair and Unchanging

Poor and Unchanging

Fair and Unchanging

Good and Unchanging

Fair and Unchanging

Fair and Unchanging

Poor and Deteriorating

Fair and Deteriorating

Good and Undetermined

Good and Deteriorating

Good and Deteriorating

Poor and Deteriorating

Poor and Deteriorating

Undetermined

Not Assessed

Fair and Unchanging

Poor and Unchanging

Fair and Unchanging

Unchanging

Increasing

Increasing

Increasing

Increasing

Sub-indicators supporting the Climate Trends assessment

Sub-Indicator

Lake Superior

Lake Michigan

Lake Huron

Lake Erie

Lake Ontario

Unchanging

Increasing

Increasing

Increasing

Unchanging

Unchanging

Increasing

Unchanging

Increasing

Increasing

Increasing

Increasing

Increasing

Decreasing

Decreasing

Decreasing

Decreasing

Decreasing

Climate information is not assessed in the same manner as other indicators in this report, as thresholds for Good, Fair or Poor have not been established at this time. Therefore, climate trends are simply assessed as Increasing, Unchanging or Decreasing. Note that the datasets used to calculate these trends span different time periods and are therefore not directly comparable.

Status

Green indicator (good status). Most or all ecosystem components are in acceptable condition.
Good
Yellow indicator (fair status). Some ecosystem components are in acceptable condition.
Fair
Red indicator (poor status). Very few or no ecosystem components are in acceptable condition.
Poor
Grey indicator (status indeterminate). Data are not available or are insufficient to assess condition of the ecosystem components.
Undetermined