Avian Community Structure Changes in a Mature Floodplain Forest after Extensive Flooding

Abstract

Effects of flooding on the vegetation and avian communities of the Bill Williams River, Arizona, were evaluated from 1976 to 1983. High water flows in 1978 through 1980 caused the death of 99% of all Fremont cottonwoods (Populus fremontii) and 64% of all Goodding willows (Salix gooddingii) on a 120-ha area near the confluence with the Colorado River. By 1982, cattails (Typha spp.) had become the dominant vegetation. Groundand canopy-dwelling avian insectivores and cavity nesters decreased, whereas passerine marsh insectivores and rail-like species increased. Water released from Alamo Dam resulted in loss of the last remaining large stand of mature cottonwood-willow habitat in the lower Colorado River valley, causing declines in numbers of some bird species possibly threatened with local extirpation. Need for flood management procedures to conserve vegetation is paramount when beleaguered habitats are involved. J. WILDL. MANAGE. 51(2):495-502 Creation of reservoirs and channelization of rivers have extensively altered plant and animal communities along many major water courses. In some areas controlled water releases in spring and early summer have resulted in a spread of native riparian tree species, as along the Platte River in Colorado and Nebraska (Williams 1978), thus increasing riparian wildlife in that area. However, in other areas, water flow management has been detrimental to both native vegetation and wildlife, as along the Missouri River (Anderson 1971, Johnson et al. 1976). Such problems are encountered also in the Southwest, where a prolific exotic tree, Chinese tamarisk (Tamarix chinensis), largely has replaced native tree species in many areas (Horton 1977). The lower Colorado River and its tributaries are such areas and serve as dramatic testimonials to the ecological consequences of heavily managed riverine systems. Examination of historical records shows that until the mid-1800's, the lower Colorado River was lined with gallery forests consisting of cottonwoods and willows. Channelization of the river, cutting of trees for firewood, fires, and land clearing for agriculture removed most of these forests (Ohmart et al. 1977). Cessation of natural floods and high soil salinities have prevented the return of these communities. Today, most Fremont cottonwood and Goodding willow areas that remain in the lower Colorado River valley are decadent and form mixed stands with exotic tamarisk, screwbean mesquite (Prosopis pubescens), honey mesquite (P. juliflora glandulosa), and arrowweed (Tessaria sericea). Only along the Bill Williams River does the cottonwood-willow forest remain relatively intact in the lower Colorado River mainstem and adjacent tributary deltas. A water release from Alamo Dam (near the confluence of the Bill Williams and Big Sandy rivers, Mohave County, Arizona) of unusually long duration flooded a 120-ha stand of cottonwood and willow on the Bill Williams River at its confluence with the Colorado River during October 1978-April 1981. Management of native vegetation is presently given very low priority in flood control procedures. This study was conducted to determine the effects of longterm flooding in a riparian forest and to denote subsequent changes in the avifauna. Results from this study will be useful in planning management options during future flooding events. Special thanks are due R. W. Engel-Wilson, M. J. Kasprzyk, J. M. Jackson, and D. Krueper for organizing and assisting with the fieldwork. D. E. Busch provided water flow data and insightful views into the workings of water management agencies. K. V. Rosenberg, A. W. Laurenzi, and V. C. Hink critically reviewed the manuscript and helped with interpolation of preand during-flood bird and vegetation data. S. M. Cook and J. R. Durham provided editorial assistance, and C. D. Zisner typed the manuscript. The research was funded partly by the U.S. Bur. Reclamation, Contract 7-07-30-V0009.