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e e e 3-9 Guidance on the erodibility of soils in a proposed channel improvement may be obtained by observation of existing channel performance. However, there should be a good general understanding of how flows will differ after project installation from what is characteristic under present conditions, that is, the change in grade, if any, the relative change in peak flows for frequent as well as infrequent events, and the relative change in duration of flows as well as frequency of bankfull flows. Observations on present stability are relevant if minor changes in flow characteristics are to be made. Such observations are substantially less relevant if major alterations in channelized flow are to result. These alterations could consist of on-site changes such as straightening and enlargement, or off-site changes such as upstream reservoirs that reduce the peak but substantially increase the duration of flows. For one example, an existing meandering channel is so small that overbank flooding may occur annually or more frequently. A proposed realigned and enlarged channel to be built to carry the ten percent event will cause a substantial increase in hydraulic stresses. The fact that existing stream banks are stable is not in itself supporting evidence that bank stability can be expected following construction. Then too, a major change in the rate of bed material load transport, if such load is present, can occur as a result of increased channel flow. While a general knowledge of changes in flow characteristics with project installed will suffice early in the field inve~tigations, specific information will be needed in evaluating the erosion resistance of soils following sampling and laboratory analysis. Bank sloughing or sliding may be a problem independent of changing dis- charge characteristics and needs to be identified. Fine sands, silts and some clays are susceptible to sloughing or slips under a range of moisture conditions. Sliding or sloughing may result from a rapid decline in water stage leaving a steep seepage gradient within the bank. The bank may slide or slough because of being undermined by erosion at the toe or by excavation below the water table. Underground flow from tributaries or springs, excessive rainfall, or over-irrigation can provide a source of ground-water flow which may result in bank sliding or sloughing. The location of any seeps or springs with reference to such problems should be identified. Reconnaissance of Site for Channel Improvements The initial field studies phase of site investigations should include inspection of the soils to and below the proposed channel invert level to interpret the geomorphology and to identify stratigraphic units. It is of primary importance to determine what stratigraphic units exist and their relationship one to another. This knowledge should be used to determine whether the same units occur at other randomly picked locations 247.9270_78 _ 4