Bedrock geology forms the template upon which fluvial landforms develop and fluvial processes act. An understanding of how geology affects geomorphic and hydrologic processes is a necessary first step in watershed assessment and management. For example, the underlying geology of a basin influences the shape and pattern of the drainage network, which in turn influences the timing of peak flows. Soil depth, composition, erodibility and rates of weathering are influenced by bedrock, and drainage density also differs by bedrock type. Consequently surface and groundwater flow regimes reflect differences in geology, which in turn can affect stream temperature regimes. The strength of hillslope-channel interactions depends on valley and channel morphology, which are influenced by bedrock geology and structure. Such interactions include the input of certain sizes and quantities of sediment to streams, the probability of the formation of channel-spanning log jams and input of riparian trees to the wood loading of a stream, and the probability of channel aggradation or bank erosion. The direct and indirect effects of geology on streams in the Pacific Northwest influence the availability and quality of in-channel habitat suitable for salmonids as well as riparian habitat needed to support large conifers.

For example, in northwestern California, drainage patterns closely follow fault zones, and channel confinement is influenced by structural controls. Earth flows tend to occur on melange units, and have rolling, subdued topography which is highly susceptible to gullying. These earth flows contribute large boulders to streams, causing steep channel gradients and narrow valleys. In contrast, drainage density is higher on schist terrain, and streams are more incised. These factors have implications for land management actions, such as road construction and maintenance practices. Attrition rates vary among rock types, so the relative contribution of fine and coarse sediments from different terrains also differs. Other examples of geologic-geomorphic interactions across many scales will also be presented.