Chapter 17: Streams and Floods: The Geology of Running Water

Drainage networks organize according to underlying bedrock structure and topography, producing patterns such as dendritic systems in homogeneous terrain or trellis arrangements where rock layers vary in resistance. The watershed concept establishes how continental divides define regions where precipitation flows toward specific outlets. Stream mechanics involve discharge, velocity gradients, and turbulent flow behavior that together determine erosional power and sediment transport capacity. Streams move sediment through three mechanisms: dissolved load carried invisibly within the water, suspended load of fine particles creating visible discoloration, and bed load of coarser material rolling or bouncing along the channel floor. As streams evolve, they produce characteristic landforms including valleys, canyons with waterfalls and rapids, braided channels in areas of high sediment supply, and meandering patterns that reflect equilibrium between erosion and deposition. Meanders migrate laterally and eventually cut off to form oxbow lakes, while floodplains adjacent to channels accumulate sediment during overflow events. Over geologic timescales, streams respond to changing base levels through rejuvenation processes, develop incised meanders, experience stream piracy where one drainage captures another, or reverse direction entirely when tectonics alter regional slope. Floods occur across a spectrum from gradual seasonal inundation to sudden catastrophic flash floods triggered by intense rainfall or glacial meltwater release. Human infrastructure including levees, dams, floodways, and hazard mapping attempts to mitigate flood risk, though urbanization, pollution, and climate variability continue to reshape human interactions with river systems and their inherent hazards.