Aquifers and aquitards along section A-B [PDF, 68.3 kb, viewer]

Aquifers and aquitards along section C-D [PDF, 50.0 kb, viewer]

A drainage basin is the entire area contributing to the runoff and sustaining part or all the flow of a main stream and its tributaries. Runoff is that part of precipitation, as well as other flow contribution, which flows over the ground surface (overland flow), infiltrates the soil surface and move laterally through the upper soil horizons (interflow), percolates deeply into the ground to become groundwater (groundwater flow), and eventually is discharged into a stream . "Catchment" and "watershed" are other terms synonymous to drainage basin. An hydrographic basin corresponds to the surface stream network. A groundwater basin or groundwatershed drains water that has infiltrated into the ground and is migrating toward a stream outlet. In many cases the groundwater basins correspond to hydrographic basins, but in areas of thick overburden and complex stratigraphy (Aquifers and aquitards along section A-B [PDF, 68.3 kb, viewer], Aquifers and aquitards along section C-D [PDF, 50.0 kb, viewer]), groundwater basins can have different extents, water divides, flow directions and outlets from hydrographic basins.

Hydrographic Basins [PDF, 4.0 Mb, viewer]

In the National Capital Area, the drainage basins located on Precambrian terrain correspond to the hydrographic basins [PDF, 4.0 Mb, viewer] as these rocks are relatively impermeable and are generally covered by only a thin layer of unconsolidated sediments. On the other hand, the Paleozoic bedrock located in south-east part of the area, the is generally covered by a layer of unconsolidated sediments that can attain several tens of metres thick (Drift Thickness) and is characterized by complex stratigraphy. Consequently, groundwater flow in this area can differ substantially from the runoff, mainly in the South Nation and east part of the Rideau hydrographic basins.

In areas of thick clay aquitards ( Maps - GeoServ ) most of the water flows as surface runoff or interflow (perched water table) when covered by sand (surficial sand aquifers). In these areas, water drainage follows closely the hydrographic basins. In recharge areas, runoff percolates into the ground until it reaches the basal till aquifer overlying the more impermeable bedrock which acts as an aquitard. In these areas, groundwater flow can follow the bedrock topography rather than surface topography, leading to major differences between hydrographic and groundwater basins in several areas. The bedrock topography map ( Maps - GeoServ ) indicates possible bedrock water divide and flow direction of groundwater at the bedrock surface.

Sources:
Brandon (1960), Charron, J.E. (1978, 1974), Chin et al. (1980), Geo-analysis inc. and J.L. Richards & Associates Limited. (1992), Owen (1953, 1953b, 1953c), Prévôt (1974), Simard and Des Rosiers (1979), Soliman et al. (1998), Williams (1991).