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24 t (2) to relocate and rephotograph the sites in the historical photos from the Monument library, (3) to rephotograph some of the photo site s from the June trip in order to document seasonal changes in vegetation and (4) to collect plants from the flood zone area. The final trip was frown September 20 to 29. The purposes of this trip were: (1) to observe the vegetation at the end of the growing season, (2) to rephotograph sites which were missed on the August trip, (3) to refine and finalize the vegetation map of the Yampa River corridor, (4) to collect any plants not previously seen or not previously in flower, (5) to collect soil samples from sand bars and beaches which exhibit successful tamarisk seedling establishment to determine whether substrate texture may be influencing reproduction of this species, and (6) to cut and age tamarisk fray the Green River and from Big Joe Rapids on the Yampa. Vegetation measurements such as those collected in August were not taken on this trip. Also in September, the Park Service, through the cooperation of the Forest Service, provided helicopter time so that we were able to photograph the Yampa canyon and parts of the Green in 35mm color slides during this low water period. The purposes of these photos were to determine the approximate area of the flood zone and to improve the accuracy of the vegetation map. The former proved to be unfeasible due to the unavailability of highly detailed, small-scale base maps of the canyon and the inconsistency of scale in the photographs. Vegetation Surveys Seven locations were selected on the Yampa River for vegetation analysis, three above Big Joe Rapids, three below, and the area of Big Joe Rapids itself. These locations are described in Table 1. Sites were selected on the basis of their potential to be stxongly influenced by high river discharges, as indicated by their physiography and the nature of their substrates. Types of areas which