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29 7 adnd bark formation must indicate that the plant is old with respect to others in the vicinity. With two exceptions, plants that were aged were from below the historic flood line. All cuts were made completely through the trunk within six inches of the ground surf ace. At Big Joe Rapids on the Yampa River, 17 plants were cut and aged. Three distinct terraces were observed here at low water, each having established tamarisk plants. Plants were cut from all three terraces. It was hoped that from this collection a correlation could be established between stem diameter and age, and therefore an effort was made to collect a range of diameter sizes from this site. Otherwise, the criteria for selection and the method of cutting were the same as for the Green. Comparative Photographs Two types of comparative photographs were used, recent and historic. The recent photo pairs were taken as color slides and show the same location in June and September (high and low flood stages) of the study period. Only the Yampa River was included in this aspect of the study. Historical photographs from the Monument library included all river corridors but the majority were from the Canyon of Lodore. The oldest were from the 1871 Powell expedition taken by E.O. Beaman and J.K. Hillers. The photo stations of this collection were relocated and rephotographed in 1969 by E.M. Shoemaker and H.G. Stephens (1969, 1975) for the US Geological Survey. Thus in some cases, we were able to produce a series of three comparative photos for a single location. All historical re-photos were taken as 35mm black-and-white. Many of the 1871 and 1969 photographs were taken with a wider angle lens than we had available. When we were unable to incorporate the entire field of view of the older photograph, our interest was in the portion showing vegetation along the river.