Drainage density refers to the frequency of streams on the land/drainage basin. In technical terms drainage density is the length of all streams divided by the drainage basin area ∑L/A (L= stream length, A= catchment area). It is measured in km/km2. The drainage density or frequency of streams is influenced by varying factors such as:
Surface nature
Permeable surfaces allow water to infiltrate through reducing channel formation which results in lower drainage densities.
Climate
Regions with high amounts of rainfall tend to have high drainage densities.
However, this is not always the case.
For example, arid regions are known to have high drainage densities despite low rainfalls. This is as a result of steep terrains associated with those regions which favour runoff and channel (river) formation. (See relief below). Infiltration is also reduced by hard impermeable surfaces.
On the other hard, rainy climates such as the tropics may not have high drainage densities if surfaces are undulating and infiltration high. Also more vegetation present there accelerates infiltration.
Vegetation cover
Vegetation impedes runoff, intercepts and stores water limiting channel formation and contribution to streams. Drainage basins with sparse vegetation cover have relatively high drainage densities.
Relief
Steep and sloppy terrains favour high amounts of runoff which triggers vertical incision gradually forming a river channel.
First, tiny channels called rills are formed. They are then enlarged into gullies which provides a channel for water to flow through.
Flat, undulating surfaces promote infiltration reducing the possibilities of a river channel formation therefore lowering the drainage density altogether
