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FLOODING PROCESSES';4ND ENVIROM\fENTS ON ALLUVIAL FANS 35 the incision. Some channel banks may be colonized by trees and bushes, adding another stabilizing influence. Because their surfaces are no longer accumulating sediment, such fans or parts of fans are said to be iI/active. Incised streamflow fans are particularly well developed in regions where a major climatic change has altered the conditions that favored sediment accumulation (e.g., the transition from glaciation to interglacial period in parts of the Sierra Nevada and the Cascade Range of Washington state, or from wetter to drier periods in the mountains of Arizona). They are also well developed in areas where fans have been steepened tectonically as in parts of southern California. In these cases, there is a strong isolation of deeply incised channels from the surrounding "fossil" fan surface. Thus the problem of recognition is complicated because all degrees of isolation occur, ranging from aggressive accumulation to deep incision. Chapter 3 describes field methods for identifying and mapping degrees of activity and for dating the time of latest activity on various parts of fans. Intermediate cases of channel stability and confinement are particularly widespread and important to recognize and evaluate. They occur, for example, in diverging channel systems (distributaries) where the sediment balance of a reach is near-steady state. Such channels may gradually become shallower downfan until their floodwaters simply disperse as sheetflood, repeatedly spreading thin layers of sediment and water and building an apron of relatively well watered and fine sediment that supports thick vegetation. In some years, there is accumulation of sediment, and in others there is net removal, so the bed may rise or fall by a few decimeters, but neither the scour nor the filling trend persists for long enough to raise or lower the channel bed significantly In relation to the fan surface. Alternatively, there may be a persistent but very gradual trend that causes the channel to rise, lower, or shift laterally at a rate that is difficult to detect with commonly available information (e.g., sequences of maps or aerial photographs, anecdotal reports, dating of recent sediments with buried artifacts). In other cases, a reach that has stabilized may be perturbed by runoff or sediment that enters it from an unstable reach upstream. Thus analysis of the stability of a reach requires taking a broad view of the potential for change in channels upfan. Spatial context is important in any analysis of flooding and sedimentation hazards on a streamflow fan. Hjalmarson (1994) provides an illustrated account of various distributary flow channels with a range offlow path stability and intensity of flood hazard. Debris Flow Fans Debris flow fans occur where strongly episodic sediment transport is triggered by collapse of an accumulation of weathered rock, soil, or sediment in a steep source region or by concentration of flow onto a steep accumulation of sediment that is then trenched rapidly in such a way that a high sediment concentration is developed with a mixture of sizes, including a significant proportion of fine sediment. The sediment-water ratio of the mixture must be so high that the flowing debris has a low permeability and water cannot drain out (upward) quickly enough to allow the water to separate from the sediment and the sediment to settle onto the bed. The resulting poorly sorted slurry is dense and highly viscous and travels as a laminar flow except where agitated by waterfalls and cascades, by larger rocks in the bed, or by engineering structures, Observers often describe such flows as looking like wet concrete. Flows with