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e e e 3-5 the weathering process has been prevented or retarded. Reduction of iron, resulting in grayish, greenish, or bluish coloration, however, can oceur. Thus sediment which may give the appearance of being recent in origin could have been deposited at about the same time as a nearby unit with a profile containing morphological evidence sugges- tive that it has been in place for a long period of time. Accumulations of or cementation by silica or carbonates, or both, can change the structure and erosion resistance of the sediments without changing the engineering classification materially because the cementing material may not noticeably add to the finer soil constituents. The junction with tributaries can change the uniformity of main stream stratigraphic units, depending upon whether the tributary has been or is a significant contributor of sediment. The type of sediment deposi- tion may be so similar to the main valley deposits that interfingering or mixing results in an accumulation that is not separately distingish- able. Tributary deposits could also be quite different either in age, color, texture or all three. Figure 3-l illustrates a geomorphic setting and related stratigraphic units that would be pertinent to proposed channel improvements on a stream and some of its tributaries. Unit I consists of recent main stream deposits and unit 2 of recent deposits from the identified tribu- taries. These deposits constitute a replacement of older alluvium that was eroded during a lowering of base level in times past. Unit 3 consists of old alluvium which has undergone aging and the development of a profile. Unit 4 is a still older buried profile and unit 5 is an old stream terrace remnant. Discontinuities in Alluvial Stratigraphic Units Discontinuities are here defined as breaks in stratigraphic units that were originally continuous. Such breaks, narrow or wide, can have a bearing on the stability of an improved channel since the deposit may be replaced by more or less erodible material. Discontinuities can occur in a number of ways. One example is illustrated by Figure 3-1 where fine grained coherent soils developed at a higher base level than exists today. A lowering of this base level, created by a lowering of the nearby sea level elevation, caused downcutting of the main stream and then its tributaries. A subsequent raise in base level induced replace- ment by more recent sediments that are erodible. Discontinuities similar to the above described type can occur without changes in base level. For example, a tributary producing a different type of sediment than the main stream can determine the characteristics of the deposit on the main floodplain in the vicinity of that tributary.