Question 18 of 21: Question 6, Part 3: Drainage Basin (Pattern) Types

Question 6, Part 3: Drainage Basin (Pattern) Types (7 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Three classic stream drainage patterns recognized from map view.

Find. A description and sketch of each.

Approach. Each drainage pattern reflects the underlying rock's structure and resistance to erosion; sketch each pattern's characteristic tributary geometry.

Dendritic. An irregular, branching, tree-like network of tributaries joining larger streams at acute angles; develops on flat-lying, spatially uniform rock or unconsolidated sediment where there is no underlying structural control on where tributaries form.

Radial. Streams flow outward in all directions from a single central high point (e.g. a volcanic cone or a domal uplift), like spokes on a wheel; reflects a locally symmetric, isolated topographic high.

Trellis. Long, parallel main streams (following the strike of weak, easily-eroded rock, e.g. shale) joined by short tributaries entering at sharp, near-right angles (cutting across resistant ridges, e.g. sandstone) – the classic pattern of folded or tilted, alternating strong/weak sedimentary belts (e.g. the Valley and Ridge Appalachians).