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, ! interpreted through detailed GIS maps. Comparable quadrats in tributaries should continue to be monitored to provide off-river controls. Continued searches for critical (endemic or endangered) and non-native plant species will be made in the range of dam operations within the river corridor. To date, no endangered plant species are at risk from dam operations in the Grand Canyon (Stevens and Ayers 1993, 1994). However, flooding may potentially increase the ranges of hydrochorous non-native species in this system. We will define and monitor the distribution of selected non-native species known to be expanding in this system, including Tamarcr pentandra, Alhagi camelorum, Lepidium /atifoliurn, Saccharum ravennae, and others identified by Stevens and Ayers (I 993,I 994). Secondary Nnll-hypothesis Ib: The marsh at river mile S5.5R will be used for detailed study of marsh vegetation response to sediment scour in coordination with teams studying eddy sediment transport processes. Before and after the high-flow experiment, basal area and standing crop measurements will be made along and maps of the topographic surface will be produced and analyzed to determine how high flows altered the surface. This component will be studied by empirical measurement of velocity changes under different flow levels in relation to pre-existing vegetation at the 55.SR study site. Pre-existing and post-flood vegetation will be measured at 55. SF. using the methodology established by Stevens and Ayers (1993) and descnOed under Secondary Null-hypothesis la (above). Sedimentology of the new deposits, and root and stem density of remaining vegetation win be measured in relation to scour patterns of vegetation. Present hypotheses of vegetation scour include the following: vegetation is lost to bar wide turbulence of sediments, floating out vegetation. .; , , 1- .;;, ~, '.. ,. ~'.: /, Null hypothesis 2: This component will be monitored on a system-wide basis using the approaches outlined in Secondary null-hypothesis la (above). Initial monitoring after the flood will only determine seedling establislunent of invasive species. Long-term assessment of non-indigenous species post-flood colonization of this system win be part of the long-term monitoring program. As part of long-term monitoring, vegetation remapping at selected eddies at selected intervals following the flood will further augment census data. Monitoring sites will also provide information on flood-related changes in the distribution of Lepidium latifolium, A/hagi camelorum, Saccharum ravennae and other non-indigenous species. t.:: '. :'.: . jt;. 1'0, j.-: .' < ;!. .. :- :;; ~; " Benthic Community and Organic Matter Displancement and Drift Obiectives The objectives of this part of the study are to understand how high discharges interact . with geomorphic influences (e,g., scouring of backwaters, reconfiguration of eddies and bars) to reset the distribution and abundance of algae and invertebrates within the entire river corridor, to 17