---
id: scenarios/great-lakes-water-budget
about:
  label: The Great Lakes' small yearly refill
  description: >-
    The Great Lakes hold about a fifth of the world's surface fresh water, but
    by Earth.com's account less than 1% of it is replaced each year, while the
    climate around them warms and their ice shrinks. How much can be drawn and
    diverted safely?
  tags:
    - Great Lakes
    - fresh water
    - water supply
    - lake ice
    - algal blooms
scenario:
  place: 'Great Lakes basin, United States and Canada'
  since: '2008'
  scale: bioregion
domains:
  - water
  - climate
  - ecology
  - people
relations:
  - predicate: example_of
    target: questions/weather-and-climate
provenance:
  origin: agent
  sources:
    - title: >-
        Scientists determine the Great Lakes are nonrenewable (Earth.com, 17
        August 2026)
      url: >-
        https://www.earth.com/earth-science/scientists-determine-the-great-lakes-are-nonrenewable/
    - title: >-
        Our Changing Climate Is Presenting Major Challenges to the Great Lakes
        Region (Wuebbles and Kunkel, JAWRA, 4 January 2026; abstract via
        Crossref)
      url: 'https://api.crossref.org/works/10.1111/1752-1688.70078'
    - title: Great Lakes Facts and Figures (US EPA)
      url: 'https://www.epa.gov/greatlakes/great-lakes-facts-and-figures'
  created: '2026-10-23'
  reviewed: '2026-10-23'
  reviewed_by: 'Claude, from the sources listed'
  status: draft
---
Lakes Superior, Michigan, Huron, Erie and Ontario hold about 21% of the world's surface fresh water and 84% of North America's, according to the US EPA.[^3] Earth.com reports that the water was left by retreating ice sheets about 10,000 years ago, that rain, snowmelt, runoff and groundwater replace less than 1% of it each year, and that hydrologists therefore treat it as effectively non-renewable on a human timescale.[^1] Industry, power plants, farms and utilities draw about 44 billion gallons a day; most returns, and evaporation, manufacturing and diversions out of the basin take the rest.[^1] Air temperatures around the lakes have risen almost 3°F since 1951, and maximum ice cover has fallen about 5% a decade, with a record low in 2024.[^1] A 2026 commentary by Donald Wuebbles and Kenneth Kunkel finds the deep water warming fastest and expects lake levels to swing further from the long-term average.[^2] The same slow turnover means contaminants stay for decades.[^1] In August 2014 a toxic bloom in western Lake Erie shut off drinking water for about 500,000 people around Toledo.[^1] The EPA counts more than 30 million people living in the basin;[^3] Earth.com says about 40 million in the US and Canada drink from the lakes.[^1] Since 2008 the Great Lakes Compact has barred most transfers of water out of the basin.[^1]

## Who needs an answer

- The states and province that share the lakes, under a Compact that bars most transfers of water out of the basin.
- The International Joint Commission, which tracks conditions under the binational Great Lakes Water Quality Agreement.[^1]
- Drinking water utilities whose intakes sit in lakes that hold on to whatever enters them.
- Shoreline towns and infrastructure owners facing higher and more variable lake levels.
- Fisheries managers, since lake whitefish eggs need winter ice cover.[^1]

## The domains it chains

Inflow, evaporation and outflow through the St. Lawrence; winter ice, which slows evaporation; warmer water that layers earlier and loses oxygen at depth; phosphorus from farm fields feeding blooms; about 190 established non-native species;[^1] withdrawals for cities, industry and power; and the law that limits diversions.

## What a twin should let people try

- Withdrawals: how lake levels respond to a given rise in consumption or a new diversion, set against a refill Earth.com puts at under 1% a year.
- Ice: what the loss of winter ice means for evaporation, lake levels and lake-effect snow downwind.
- Blooms: how cuts in farm phosphorus change the size of Lake Erie's summer blooms and the risk to drinking water intakes.

## Where to start

NOAA's Great Lakes Environmental Research Laboratory keeps the ice cover records back to 1973,[^1] and the EPA's Great Lakes pages give the basin's physical figures.[^3] The [NOAA National Water Model](/engines/noaa-national-water-model/) simulates streamflow in US rivers, including those that feed the lakes.