Question 9 of 9: Drainage Patterns

Question 9: Drainage Patterns (10 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.

Dendritic drainage is a branching, tree-like network of tributaries joining larger streams at irregular (typically acute) angles, with no strong preferred orientation. It develops where the underlying rock or sediment is relatively homogeneous and flat-lying – uniform resistance to erosion in every direction (horizontal sedimentary strata, homogeneous granite or crystalline basement, thick uniform till/glacial drift) lets streams erode headward wherever slope gradient happens to steer them, with no structural grain to bias their direction.

Radial drainage is a pattern of streams flowing outward in all directions from a single central high point, like spokes from a hub. It develops on an isolated, roughly conical topographic high – classically a volcanic cone or a domal structural/intrusive uplift (laccolith, resurgent dome) – where the composition need not be special, but the landform geometry (a symmetric high point with slopes radiating outward on all sides) forces streams to diverge outward from the summit regardless of the underlying rock type.

Trellis drainage is a rectangular pattern of long, parallel main streams with short tributaries joining them at close to right angles, resembling a garden trellis. It develops in folded (or otherwise structurally banded) sedimentary terrain with alternating bands of resistant and non-resistant rock aligned in parallel ridges and valleys (classic Appalachian-style folded strata): main streams follow the long strike-parallel valleys eroded into the weaker beds, while short tributary streams cut directly across the resistant ridge-forming beds along joints or wherever they can breach them, joining the main valley streams at sharp angles.