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Great Basin Naturalist

Abstract

In March 1991 a 10-year return flood (368 m3s−1) occurred in the Hassayampa River, a perennial stream (0.1 m3s−1 base flow) within the Sonoran Desert. Depth of the floodwater ranged from 2.64 ± 0.20 m (mean ± SD) near the stream to 0.47 ± 0.31 m in the highest floodplain zone (Prosopis forest). Flow velocity was 1.7 ± 0.6 m s−1 and 0.9 0.4 m s−1 in these same zones. An average of 8 cm of sediment was depositred on the floodplain, with maximum deposition (to 0.5 m) on densely vegetated surfaces 1–2 m above the water table. Native riparian vegetation showed resistance and resilience to the flood disturbance. Plants on high floodplains (e.g., Prosopis velutina trees and saplings, and Populus fremontii and Salix gooddingii trees) had low mortality. Populus fremontii and S. goddingii "pole" trees and saplings were on less aggraded floodplains and sustained varying mortality depending on floodplain elevation and depth of flood waters. For example, P. fremontii pole trees on 1–2-m-high floodplains averaged 6% mortality, compared to 40% for those on low floodplains (<1 m above the water table) where standing water was >2 m. Seedlings of Populus fremontii and Salix gooddingii established abundantly after the flood along overflow channels and main channel sediment bars, contributing to age-class diversity for these episodically recruiting species. The exotic species Tamarix pentandra had greater mortality of pole trees (62%) and low post-flood recruitment compared to P. fremontii and S. gooddingii. Survivorship of shrub species also corresponded to floodplain elevation. Zizyphus obtusifolia grew on high-elevation floodplains and had no mortality. Shrub species of lower-elevation floodplains underwent mortality but revegetated after the flood via asexual reproduction. For example, stem density of the dominant shrub (Baccharis salicifolia) declined by half but recovered to pre-flood levels by late summer primarily via stem sprouting. Dominant herbaceous plants on stream banks and low floodplains (i.e., the rhizomatous perennial grasses Paspalum distichum and Cynodon dactylon) similarly compensated for a 50% decline in cover by vegetative spread. The post-flood herbaceous understory vegetation in high-elevation floodplain zones (i.e., Prosopis velutina forests) remained sparse throughout the summer and shifted in composition from nearly monotypic stands of exotic annual species to more divers mixtures of native and exotic annual grasses and forbs.

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