Drainage Basins

A drainage basin, or catchment as it is also called, is the part of the land surface that is drained by a single river system. Most precipitation that falls on a drainage basin will eventually reach a main river and flow to the sea. A drainage divide, on either side of which water will flow to separate basins, marks the perimeter of a catchment. The characteristics of such a basin, that is, the size, shape, vegetation cover, soil, and bedrock, determine how fast rainfall reaches a main river and the frequency and severity of flooding.

Diagram Represents a Typical Small Drainage Basin (Catchment) in Newfoundland and Labrador (Idealized)
Diagram Represents a Typical Small Drainage Basin (Catchment) in Newfoundland and Labrador (Idealized).
The main stream drains WSW and is fed by small tributary streams, by downslope seepage of water in the soil (soil water) and bedrock (ground water), and by springs emerging in the stream bed (not shown). Tributaries drain from small ponds occupying bedrock hollows eroded by glacial ice, or depressions in glacial deposits. Wetland has developed in other depressions.

Other features are also the result of glacial action. The largest of the ponds (lake) has a long narrow shape and fairly regular outline typical of concentrated glacial erosion in a pre-glacial valley. Widespread erosion by the ice sheet eroded soil and weathered rock, exposing the bedrock in rock outcrops. Some of these are on the higher ground, near the divide; others may occur in the stream's course, creating falls or rapids. Glacial erosion on the divide created a depression now occupied by a lake which drains two ways - into the basin and into the adjacent catchment.

Divide: the boundary between one drainage basin (catchment) and the adjacent basin.
Downslope seepage: downslope movement of groundwater close to, or at the surface.
Fall or rapid: steepening of a steam's gradient over an outcrop of bedrock.
Lake in glacial rock-basin: a natural water body occupying a rock-basin eroded by glacial ice into bedrock.
Main stream: the stream into which all the surface water in the catchment eventually flows.
Ponds: small water bodies.
Rock outcrop: bedrock at the surface, normally the result of glacial erosion.
Tributary: a stream draining into a larger stream.
Wetland: bog, fen, marsh; an area of poor drainage where poorly decomposed plant material accumulates to form peat.

Adapted and colourised by Duleepa Wijayawardhana from a diagram in The Natural Environment of Newfoundland, Past and Present. Edited by Alan G. Macpherson and Joyce Brown Macpherson (St. John's, NL: Memorial University of Newfoundland, © 1981 MUNCL) 184.

Catchments in Newfoundland and Labrador are fairly typical of areas, with resistant (strong) bedrock, that have been overrun by glaciers. As a result, they have numerous lakes and wetlands and variable slopes. Two types of lakes are common: small, shallow, irregular ponds and larger, deep rock-basin lakes, which may also be called ponds. Western Brook Pond, a large lake in Western Newfoundland, is an example. Small ponds can occupy hollows, either eroded in the bedrock by glacial action, or created by the irregular surface of glacial deposits. Damming due to natural accumulations of glacial, fluvial or marine deposits or even beaver dams may also create ponds.

Rock-basin lakes are usually long and narrow, reflecting the local bedrock characteristics (e.g., faults) and the direction of glacial movement. All the major lakes of the province are of this type. Artificial dams, such as those at Red Indian Lake and Grand Lake on the island's west coast, have enlarged other lakes.

Variable slopes in a drainage basin are most evident along a river's course; steep gradients cause rapids or waterfalls and low gradients form pools or areas of slow currents called steadies. Dramatic waterfalls arise when streams draining into a rock-basin lake fall from a great height as at Western Brook Pond. The energy of a stream falling over a considerable drop can be harnessed to develop hydroelectricity. Churchill Falls in Labrador, Deer Lake in western Newfoundland, and Grand Falls in the centre of the island, have all been used to create hydroelectric power.

The irregular surface produced by glacial activity locally creates poorly drained hollows in a drainage basin; these develop into wetlands (bogs, fens and marshes). Here water logging prevents the decay of dead plant material that as a result accumulates as peat. The southern Avalon Peninsula has so much precipitation that wetlands are very extensive, forming blanket bogs.

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