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5 r ~ River, the determination of the roles of natural flow and water level fluctuations on the stream morphology and the ecology of the riparian _ and floodzone vegetation. The contrast to the physical and biological changes which have occurred on the Green since the construction of the dam at Flaming Gorge will provide an objective basis for Predicting the changes which would occur on the Yampa River should an impoundment be - - - , made upstream. ~' The critical area for potential change on the Yampa is the zone `;~- ~~`~~ / cv~ ~E~~~ ~` i ua~-'_. below the current natural maximum flood level and above the regulated ~%` '~,,~~ ~- '~~~~ / i ~~~ maximum discharge level, were a hydroelectric dam constructed upstream. Potential vegetational invasion of this zone will depend to a large extent on the substrate materials exposed and the relationship to new..- ' r varying water levels as a source of soil moisture. To accomplish this objective, several lines of study have been pursued on all three river corridors, with principal emphases on the Yampa and upper Green, respectively. Detailed studies were made of selected areas deemed representative of situations most sensitive to river level fluctuations (beach areas, back eddies, and gravel islands). Special attention was given to conditions present and plant invasion response in the zone between high flood level and the low level (the floodzone). Consequently, sampling was scheduled for low water conditions of August and September. Relief and substrate types were monitored in coordination with vegetative cover and density measures. Soil texture characteristics were measured in relation to areas of tamarisk seedling establishment. Establishment history of tamarisk on both the upper Green and the Yampa was further investigated by dating stems of mature tree specimens.