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e e .~ I ! w, ~ 1494 W, L. GRAF tape or were surveyed for comparision with similar measures made photogrammetri- cally as checks on photo measures, Land- mark measures were made from vertical aerial photos taken with mapping cameras to check the accuracy of ground photos for the 5 sites not surveyed in the field, Sediment samples from the river bed, ephemeral forms, forms stabilized by tamarisk, and alluvial deposits above pres- ent flood levels provided information about materials at 5 sites along the last 120 km (75 mil of the Green River. Minerological analysis of the samples was accomplished with low-power magnification. Trees drilled with a Swedish increment borer I I ._,-,-. -1'-'-'-'-1 . , ! I I SALT LAKE . I · I I I D A H 0 NEVADA UTA H o I o MI. ............. , '-..... .............. 200 I ,'-- .-L.~ I 100 KM. Figure 6. Colorado River basin, adapted from U.S. Bureau of Reclamation (1946). provided sequences of tree rings for age d. termination of the surfaces on which tI1 trees were growing. RESUL TS Spread of Tamarisk An accurate assessment of the gene rate of spread and data of introduction specific areas for tamarisk is a prerequislj to a clear understanding of the impact o~ the plant on geomorphic systems, By OJ taining photographs taken during sever, different years at the same site, I was able , identify the approximate year of ramaos invasion for the site in question. 1n mo~ cases, the precise, year c~uld not b,e ide~1 nfied, but the period dUring whIch Il1fesra'j tion occurred could be narrowed to 4 yr Ol! less by identifyng the latest photo WithOWI,i tamarisk and the earliest photo with L~ plant, By plotting the middle years of suJil periods for a number of frequenth- Phot~1 graphed sites against an arbitrary distan(l measured along the Colorado and Greeo,' Rivers, an accurate record of the spread 011 the plant developed (Fig, 8; Table 2) 'I Tamarisk spread through the study area Int and near Canyonlands National Park bel tween ]925 and ]931. ' Tamarisk spread consistently throughoU1 the Colorado and Green systems withoUI responses to the installations of reservoir> unlike the situation described by Harri! (1966) for other southwestern drainag< areas, A simple linear regressionberwetO time and distance shows that the planl spread generally upstream at about 1~1 km/yr (12 mi/yr) from the sourhwesteO:j part of the basin, The annual prodwction ori 600,000 windborn seeds for each lllatuf1i tamarisk tree accounts for the rapid rate oil spread (Robinson, ]958), ' The invasion chronology of tamarisk isl significant because it indicates that if thel fluvial I3ndforms were in equilibrium Wl!~! stream discharges before the arriv:;ll of the! pia nt, and if new states of equilibriulll were! established after the arrival of the plant, the! new states have probably prevailed longesti in the lower reaches of the Colorado Basin" New states of equilibrium probably ap- peared most recently in reaches lIPstreal!1[ from the confluence of the Co!orado ana: Green Rivers. Geomorphic Responses t The commonly occurring terrace 3 to 51!1V (9 ro 15 It) ,hove low-w"" Iml h""J