The relation between mainstream length and area in drainage basins

Hack and Gray have both found that data on mainstream lengths and basin areas can be represented by the equation L′ = CAn′, where C is about 1.4 (measurements in miles), and n′ is about 0.6. Hack has suggested that the deviation of n′ from ½ can be explained by an elongation of drainage basin shapes with increasing area. We point out that variations of stream sinuosity with area can also affect the value of n′. A simple approximate formula is deduced that relates mainstream length, sinuosity, basin shape, and area, and shows how the contributions of sinuosity and shape variation to the quantity (n′ − ½) may be separated and evaluated. Data that illustrate these points are presented for both real and simulated stream systems. Our conclusions are: (1) Variation in mainstream sinuosity with area can be responsible for a significant part of the deviation of n′ from ½. (2) The generally accepted statement that drainage basins become more elongated as their area increases needs further investigation.