Question 6 of 7: Surface Runoff & Stream Processes

Question 6: Surface Runoff & Stream Processes (20 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.

(a) When rainfall begins, water first infiltrates the soil. As rainfall intensity persists and the soil wets up, the infiltration capacity – the maximum rate at which the soil can absorb water at a given time – declines; once rainfall intensity exceeds it, excess water begins to pond and move as a thin, unconfined sheet flow across the ground surface (Hortonian overland flow). Sheet flow is inherently unstable: small irregularities in micro-topography concentrate it into narrow, ephemeral channels called rills, the first stage of channelized erosion. As rills capture more of the flow and erode headward and downslope, they coalesce into larger gullies and eventually into established stream channels, so runoff evolves from unconcentrated sheet flow to progressively more concentrated, higher-energy channelized flow with distance and time from the onset of a storm.

(b) (i) Alluvial fan – a fan- or cone-shaped body of sediment (often coarse) deposited where a stream emerges from a confined canyon onto a flatter plain, loses velocity, and spreads its load radially. (ii) Delta – a sediment deposit (named for its triangular, Greek-letter-Δ-like shape) formed where a stream enters standing water (ocean or lake), abruptly loses velocity, and deposits its sediment load. (iii) Distributary – a channel that branches off and carries water away from the main channel (the opposite of a tributary, which flows into it), typical of deltas and alluvial fans where a stream splits into multiple channels.

(c) The source-to-mouth cross-section of a stream is its longitudinal profile (i). Stream gradient is typically steeper near the source than at the mouth (ii), because elevation drop is concentrated in the headwaters while the lower reaches flatten out. A stream lengthens its course by headward erosion (iii); when a headward-eroding stream cuts back far enough, it can breach and capture the drainage of an adjacent stream, a process called stream capture (piracy) (iv). Where a river valley cuts through a ridge or mountain that lies in its path, the resulting landform is called a water gap (v).