REFERENCE BASE
SURFACE WATER

O.T. FORD

Water exists on earth in two basic states: ground water and surface water. There is no beginning to the hydrological cycle, but we can start with water in the air. Water vapor condenses in the air and falls to earth. At that point it can do one of three things: evaporate again; soak in and become ground water; stay on the surface and become surface water. Surface water is still subject to gravity, and after it builds up, it will start rolling downhill, taking the route that leaves it, at any point, as low as possible.

A drainage path is any such route followed by surface water as it flows downhill. The terms ‘river’, ‘stream’, ‘creek’, ‘ravine’, and ‘waterway’, and in certain contexts ‘valley’, ‘canal’, and ‘channel’, all describe drainage paths. Fundamentally there is no difference between drainage paths with very little flow and those with great flow; each still represents water flowing downhill. Even temporarily-dry paths are still fundamentally the same, as all flows vary.

A settlement basin is the lowest point, furthest downstream, in which surface water collects. This includes the ocean as well as certain other bodies, such as the Caspian Sea and the Great Salt Lake. The named “oceans” on Earth are all in fact one large settlement basin, containing most of the world’s surface water. The conventional oceans as taught in the United States are the Pacific, Atlantic, Indian, and Arctic; other systems add a Southern Ocean, ringing Antarctica. These distinctions make sense in certain local contexts, where conventional oceans lie in opposite directions (Panamá, for example). But where conventional oceans come together, such as southern Africa (where the Atlantic and Indian Oceans meet), the distinctions are seen to be clearly arbitrary.

A drainage basin is a land area from which surface water flows into a particular drainage path or settlement basin. For example, all the water that falls in the Mississippi River drainage basin will eventually drain into the Mississippi. In modern usage, this is generally called a ‘watershed’. In original usage, a watershed was the ridge that divides water that flows into one drainage path or body of water from water that flows into another; it was, in other words, the elevated border of a drainage basin.* As the term ‘watershed’ has come to be used for the drainage basin itself, the dividing ridge is now called, in that context, a ‘watershed break’. A continental divide is a watershed on any large landmass separating the drainage basins of opposite sides of the landmass, particularly of different conventional oceans, as for example the North American continental divide separates water draining into the Atlantic from water draining into the Pacific.

Internal drainage is the process of surface water collecting in any settlement basin that is not the ocean. A continuous area of internal drainage need not have a single settlement basin; two or more adjacent internal drainage basins may form a larger area of internal drainage. The largest body of water formed by internal drainage is the Caspian Sea. The Great Basin in North America is an area of internal drainage; the largest settlement basin within the Great Basin is the Great Salt Lake.

A pool is a basin in the middle of a drainage path. Most “lakes”, such as the North American Great Lakes or the African Great Lakes, are pools. The key feature of a pool is that water flows in and then eventually flows out; it is formed by a relative high spot, a dam, that obstructs downhill flow, until enough water collects behind the dam to overtop it, at which point water begins flowing downhill again. A reservoir is thus a kind of pool, an artificial pool behind an artificial dam.

There are four main pools in the North American Great Lakes, upstream to downstream: Superior, Michigan-Huron, Erie, and Ontario. Michigan-Huron is conventionally divided into two lakes, of course, but this is based more on tradition and visuals than hydrology. In fact, water flows into this large pool from all sides, but then back and forth freely between the two lobes, maintaining a single water level. Any argument that sees Michigan as separate from Huron must also see Georgian Bay as separate from the rest of Huron. The main downhill flow of the waterway leaves Superior and enters the main part of Huron, and then exits the main part of Huron at the opposite end.

* The metaphorical use of ‘watershed’ still refers to the original sense: “a watershed moment in history” (that is, a time or event that divides history into two different periods); “the 9 PM watershed” (the time before which and after which very different television shows are acceptable).

© O.T. FORD