Major Drainage Areas (MDA), as portrayed in this map, are part of the Standard Drainage Area Classification (SDAC) which was developed by Natural Resources Canada, in partnership with Statistics Canada, and Environment and Climate Change Canada.

A drainage area, as opposed to a drainage basin or watershed, is a unit of statistical aggregation used for reporting purposes based upon drainage patterns and the underlying basins, delimited by a drainage divide.

A drainage basin, sometimes called a watershed, is an area where all surface water shares the same drainage outlet. Surface water consists of water flowing on the surface of the earth that develops into larger streams and eventually combines to form a river.

The 11 MDAs presented in this map comprise the uppermost level in the drainage area classification system which then divides into 164 sub-drainage areas, and then is further divided into 974 sub-sub-drainage areas.

Relief was derived from the merging of two raster data sets, both resampled to 804 metre resolution:

- High Resolution Digital Elevation Model (HRDEM) – CanElevation Series

- Canadian Digital Elevation Model, 1945-2011

Further information on all these maps can be found on the Atlas of Canada web site atlas.gc.ca.