Diel drift/colonization studies were conducted in Sycamore Creek, Arizona, U.S.A. in late June, 87 d post-spate and, again, 4 d after recession of a large mid-August spate to examine differences in benthic algal colonization of space cleared by small-scale, localized disturbance versus large-scale, system-wide disturbance. Cell densities and species composition in the algal drift pool were quantified with hourly samples, composited every 3 h for 36 h. Diel changes in colonization activity were assessed by sampling ceramic tiles incubated during each of the twelve 3-h periods (instantaneous colonization). Cumulative changes in algal communities were assessed by collecting tiles, introduced at the start of each study, at 3-h intervals for 36 h, then again at 48 and 96 h. Consecutive 3-h measures from instantaneous-colonization samples were summed to compute expected cell densities of taxa within different algal divisions, based on short-term colonization alone, after 6 to 36 h, and compared to actual cell densities on cumulative tiles. Tiles introduced during the June interflood period were colonized much more slowly than in August, and supported assemblages dominated by bluegreen algae. Comparisons of cell densities and changes in taxonomic structure in instantaneous- and cumulative-colonization assemblages indicated that, in June, algal reproductive activity was low and rates of emigration and death were high. In August, diatom and green algal densities in drift and colonization assemblages were significantly higher than in June, and accrual of these taxa on cumulative-colonization tiles exceeded that expected, indicating rapid reproduction. August drift and colonization assemblages exhibited clear diel changes in both cell densities (with mid-day maxima) and taxonomic structure, suggesting that rates of immigration, emigration, and reproduction varied among taxa. In June, mid-day peaks in algal drift activity were caused by passive entrainment of cells into the water column, likely by oxygen bubbles produced by photosynthesis. In August, live diatom cells exhibited higher drift peaks than dead cells, indicating reproductive activity and, likely, changes in cell buoyancy was influential in generating drift maxima. Many diatom taxa displayed clear mid-day minima in colonization efficiency, corresponding to periods of peak drift, suggesting diel changes in cell bouyancy. No such patterns were detected in bluegreen or green-algal taxa. Rapid recovery of diatom assemblages in Sycamore Creek after spates appears to be driven by rapid reproduction, emigration, and reimmigration of early successional diatom taxa. The rate and pattern of benthic algal colonization of open space in Sycamore Creek, and the mechanisms controlling this process, varied considerably depending on the circumstances under which substrata became available.