Working to Protect Ontario’s Headwaters, Natural Heritage, and Watersheds


The OHI describes OHMapping as a superficially meaningful tool. Although simple, it depicts both the state of our existing natural heritage and the extent of human impact upon it, from the relative absence of regional forests and wetlands to where watercourses that have been straightened or put underground.

The project has been developed to date with guidance from GIS expert and OHI board member Rebecca Ma, with most of the hard slogging on data done by students in the  GIS program at Sir Sandford Fleming College.

The project currently focuses on three outcomes, although more are on the drawing board. First, the watercourses are colour-coded by stream order, allowing people to better find and follow the flow of the water. Secondly, the combined catchment areas of first and second-order stream are mapped, allowing people to visualize these areas. Finally, some aspects of how a each watershed’s headwater areas contribute are provided in a summary box beneath each map.

Readers should be aware that the summary data on headwater catchments, stream length, and wetlands are based only on first and second order streams. While most scientific literature includes third order streams in the definition of headwaters, the OHI has selected a tighter area of focus due to the extent of development, the predominance of private ownership, the lack of current data on the condition of headwaters, and the need for enhanced stewardship for headwater health across southern Ontario.

Readers should also be aware that the images and other information depicted in the data were assembled in the early 2000s, and do not reflect recent changes. In particular, forest and wetland cover may have changed significantly in the intervening years. From our perspective, we believe that Ontario needs to allocate additional resources to in order to have more up-to-date photographs; to adopt mapping protocols to highlight changes (we have been told that one third of the wetlands shown in the Credit photos no longer exist); and to develop indicators that can track key aspects of headwater health in both watershed report cards and assessments of the effectiveness of initiatives such as the Provincial Policy Statement and the Greenbelt Act.

The OHI does not have all the answers, nor do we advocate a blank cheque approach to solving all the problems. We do, however, advocate a robust discussion regarding expanded GIS-based Natural Heritage inventories and assessments, compatible formats for file-sharing between agencies, and easy public access to timely, meaningful data.

Before you begin to explore the maps, please take a moment to look at the image and the types of impacts described below it.


OH Mapping Snapshot

1. A large area of agricultural land. Without any streams visible, this area is probably in drainage tile.
2. Straightened first (green) and second-order streams (blue), also lacking much streamside vegetation. These watercourses offer low ecological value compared to meandering streams, while the absence of riparian plants may result in thermal loading.
3. Example of historic stream mapping on a more recent photo, showing streams running through houses where they have obviously been piped underground. Results include diminished natural habitat and ecological values, reduced exchanges with groundwater, and dank water flowing downstream.