THE ORIGIN OF VALLEYS, A FRIENDLY AMENDMENT TO PLAYFAIR'S LAW

Playfair’s “law” (1802) assumes a valley is “the work of the stream that flows in it.” However, Dury (1953) calculated that most streams are “underfit,” with valleys 3 to 10 times larger than could have been made by the streams in them.

This study of symmetrical drainage patterns and landscape proposes to amend Playfair’s law as follows (underline):

”Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together forming a system of vallies, communicating with one another, and having such a nice adjustment of their declivities that none of them join the principal valley, either on too high or too low a level”

“This improbable circumstance suggests valleys are formed two and four at a time by some over-arching process or underlying structure. Sub-glacial erosion appears to be responsible.”

Forces within flowing ice determine where basins divide (in the center) and where streams form (near the edges). Resulting drainage basins are essentially cirques, and tend to be symmetrical in shape and area with “efficient,” stable channels.

Shear stress near the edges of a lobe of flowing ice is much higher than in the center. This results in a stream forming down either edge and a ridge or drainage divide in the center. Eventually this forms two basins with two lobes of flowing ice. Now a drainage divide forms in each of these sub-basins.The result, on any substrate, is landscape symmetry and rivers that branch into two or four tributaries.

Drainage patterns in cirques are often miniatures of drainages hundreds of square kilometers larger. There is one basic pattern for these drainages, but like snowflakes, they are all different. The Mississippi/Missouri valley “system” shows this cirque-like pattern.

The symmetrical drainage pattern produced when a glacier is stable for a sufficient time, is a document of glacial periods, sea level changes and other climate related events.