Cumulative Impact Assessment of Withdrawals, Consumptive Uses and Diversions: 2016-2020

Download the report.

In the Great Lakes-St. Lawrence River Basin Sustainable Water Resources Agreement (Agreement), the Great Lakes and St. Lawrence River States and Provinces (the Parties) commit to periodically assess the cumulative impacts of Withdrawals, Consumptive Uses and Diversions of Water from the Great Lakes—St. Lawrence River Basin (Basin). Similar commitments are included for the Great Lakes States in the companion Great Lakes-St. Lawrence River Basin Water Resources Compact (Compact). As required by the Agreement and Compact, the cumulative impact assessment will be conducted for each Lake and St. Lawrence River watershed and for the entire Basin. The assessment fulfills the requirement of the Compact and Agreement.

The assessment will be used for a review of decision making standards and their application, and
for other purposes. The Basin water budget is an accounting of water flows into and out of the Basin. Some of these flows are natural and some are constructed or affected by humans. Withdrawals, Consumptive Uses, and Diversions are part of the water budget. These flows vary from year to year, either due to variability in climate or due to human activities. The timeframe for this assessment is 2016-2020. For comparative purposes, longer data sets for flows are presented to provide a historical
context for 2016-2020 data. The longer data sets are 1950-2015…

A significant addition to this Cumulative Impact Assessment report is a more robust consideration of uncertainty in historical water balance components, and of the extent to which historical water balance components might have been impacted by climate change. It is important to note (as indicated in previous reports) that not only is the magnitude of historical water balance components much greater than that of diversions and consumptive uses, but also that the uncertainties in historical water balance components are often greater than the cumulative effects of diversions and consumptive uses. To address this challenge, a new analysis framework was developed for the Great Lakes Basin that uses statistical methods to solve a basin-wide, lake-to-lake water balance model. This new modeling framework, which is presented in detail in a Supplementary Report titled, “Analysis of Great Lakes Water Balance Components,” leads to water balance component
estimates with significantly reduced uncertainty.