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36 ALLUVIAL FAN FLOODING intermediate sediment-water ratios and characteristics between those of debris flows and turbid water flows are sometimes referred to as hyperconcenlrated flows. Debris flows consist of the full range of sediment sizes supplied from the source area, and flows generated from rocks of different types within the source basin may contain different proportions of clay. The greater the proportion of fines, the greater is the internal strength of the flow because of "cohesive" bonding caused by electrical charges shared between clays and water films. Some flows are sufficiently dense and viscous to transport boulders; others leave the largest boulders behind. As the sediment-water ratio decreases (i.e., in more dilute flows), progressively smaller boulders settle to the bed and are deposited or transported as traction load in a turbulent flow. The flow properties of the slurries determine the fate of the debris flows when they emerge onto the fan, the nature of sediment deposition, and the resulting morphology of the deposit. These properties depend on the magnitude of the discharge and the rheological properties of the debris, which in turn are controlled by its sediment -water ratio and clay content. Discharge rate, clay content, and sediment-water ratio of each debris flow are set by the generating mechanism and the particular combination of circumstances that trigger the flow. For example, a large rainstorm or snowmelt may generate landslides that fall into stream channels containing significant discharge, and the resulting mixture may produce a dilute debris flow. Collapse of wet debris into a steep channel network that already contains a large volume of fallen debris from centuries of slow mass wasting on adjacent hillslopes may result in scour of that accumulation into a particularly dense and viscous, boulder-charged debris flow. The volumes and peak flow rates of debris flows depend on (I) the magnitude of the water supplied from a rainstorm, snowmelt, lake outburst, or volcanic eruption, and (2) the volume ofloose debris that is available to be liquefied by this water during the initial collapse, undermining and assimilation, or scour from the valley floor along the steep portion of the debris flow track. Thus, the debris flows that supply and mold. anyone fan have a probability distribution of discharges and rheological properties, which determine the nature and magnitude of flood risk. Fortunately, these aspects of flood risk can be read from the morphology of the fan and its source basin. The range of rheological properties among debris flows emanating from the source valley usually accounts for differences in morphology on different parts of a single debris flow fan, Flows with the highest sediment-water ratios and therefore the greatest strength come to rest on relatively steep gradients (typically 6 to 8 degrees) on the upper parts of the fan in the form of bouldery snouts and levees. These deposits block channels scoured by water floods between debris flow episodes and divert later flows of water or debris into new channels. The result is a topographically rough surface of berms, lobes, and bouldery channel blo'ckages on the upper parts of debris flow fans (Figure 2-5). Somewhat more dilute and weaker flows travel through the steepest channel reaches, but deposit bouldery levees as their margins are slowed. If the peak discharge rate of a debris flow exceeds the conveyance capacity of the channel, its upper part is partially decanted overbank and it travels some distance across the fan surface until it becomes slow enough and thin enough to stop as a bouldery or gravelly sheet with a sharp edge. Stranding of boulders in levees and overbank sheets causes a progressive downfan reduction in the boulder content of flow deposits. The most dilute and weakest debris flows remain channelized as far as the lower parts of the fans, where gradients may be as low as 2 to 3 degrees. Some of these flows halt within the channel, raising its bed and lowering its depth, while others spread over the banks onto the surface of the