Great Lakes Environmental Assessment and Mapping Project

Tributary dams

The installation and management of dams threatens the diversity of native Great Lakes fish by restricting or eliminating connectivity between the lakes and critical spawning, nursery, and overwintering habitats. Tributaries also provide water, nutrients, and sediment to the Great Lakes, particularly to coastal and near-shore ecosystems. Although limitation of fish passage and denial of fish access to the upper watershed are important consequences of dams, these influences are difficult to quantify and so the influence of tributary dams as a stressor is limited in this analysis to those effects associated with altered delivery of sediments, nutrients and organic matter.


Mapping tributary dams as a Great Lakes stressor

  • Dam data for Ontario were acquired from the Ontario Ministry of Natural Resources (OMNR).
  • U.S. dam data were derived from the National Anthropogenic Barrier Dataset developed through a project sponsored by the U.S. Fish and Wildlife Service (USFWS).
  • At least 12,000 dams occur throughout Great Lakes basin; of these ~3,400 are considered “large” (> 2 m in height), including1,910 in Canada and 1,452 in the U.S. This reduced list of dams >2 m is considered most likely to influence nutrient and sediment regimes via impoundments.
  • The influence of watershed dams as a stressor was estimated using river tributaries as pour points, and the magnitude of the influence at the pour point was scaled to the count of dams per watershed.


Number of large dams per major Great Lakes watershed



The influence of dams was then propagated into the Lakes assuming their influence decayed to 10% of effect within 15 km and became negligible at 30 km, identical to nitrogen and phosphorus propagation.

Spatial distribution of tributary dams as a stressor in the Laurentian Great Lakes. (Inset: Southern Georgian Bay, Lake Huron).