figure 1.3 The forested banks of the Amazon River.
Drainage patterns The pattern of streams and rivers within a catchment can vary greatly. Often there are similar characteristics based on the underlying geology and structure of the drainage basin. Here are four common types: • • • •
Dendritic – a tree-like pattern where water may converge (meet) from a variety of directions before joining a main river channel. Rectangular – where the streams and channels follow geological weaknesses and gaps in blocky bedrock. Radial – where water drains away from a central high point, hill or mountain into separate channels. Trellised – where streams follow slopes downhill and converge along areas of eroded rock.
Endorheic drainage basins Endorheic drainage basins are inland basins that do not drain to an ocean. Instead their base level is an inland lake or sea. Around 18 per cent of all land drains to endorheic lakes or seas or sinks. The largest of these consists of much of the interior of Asia, which drains into the Caspian Sea, the Aral Sea and numerous smaller lakes. The drainage basin is known as an open system as water is not confined to a specific location and can move from one state to the next at any given time. The different stages are explored in Figure 1.5 in a simplified systems diagram.
Dendritic
fractures Rectangular
Radial ridge valley
Trellised figure 1.4 Drainage basin morphology
Hydrology and fluvial geomorphology 11