/ Because of the interdisciplinary requirements of studies of river-floodplain systems, development of this field in the United States has been slow, and much information needed for watershed and river-basin planning is not available. This is particularly true in the southwestern United States, where study has been further complicated within the last 50 years by the introduction and spread of saltcedar (Tamarix chinensis)which has occurred simultaneously with other independently generated environmental changes.The spread of saltcedar has been aided both by purposeful planting and by a fortuitous combination of events that has weakened the native ecosystem at the time that seeds ot the new species have been made available, events that possibly include changes in such environmental parameters as flood frequency, channel stability, the season of the peak annual flood, water temperature and salinity, and sediment grain size. Careful research is needed to unravel and understand the network of relationships involved.Phenomena are arranged in chains of necessary sequence .... If we examine any link of the chain, we find it has more than one antecedent and more than one consequent .... Antecedent and consequent relations are therefore not merely linear, but constitute a plexus; and this plexus pervades nature. (Gilbert, 1886, p. 285-286).

No abstract

Like many dryland rivers of the southwestern United States, the central Rio Grande suffered a collapse of its native cottonwood forests and an expan.~ion of tamarisk (Tamarix spp.) in the early 20th century. A paramount example of an opportunistic colonizer, tamarisk occupied land made available by the plow, the bulldozer, and the shrinking of a channel depleted of flow by upstream water development. Changes in both the physical environment and the native vegetation were well underway by the time tamarisk became widespread. There is no evidence that it actively displaced native species nor that it played an active role in changing the hydraulic or morphologic properties of the river. Its present donfinance in the Presidio Valley is due to the chance conjunction in 1942 of a large summer flood, a seed source, and declining cotton prices that fostered abandonment of farm fields. The history of tamarisk on the central Rio Grande demonstrates the complex nature of vegetation change. The passive role of tamarisk in landscape change holds the hope that its response to geomorphie and hydrographic variables can be understood and predicted.

No abstract

A regeneration of thickets of Fremont cottonwood along the middle portion of the Fremont River, Utah, followed the spring snowmelt flood of 1973. The 1973 spring flood was the largest May discharge in 25 years of record at Caineville Gage. Although smaller than late season floods, the 1973 spring flood was sufficiently large to cause channel scour and overbank deposition, and occurred during the floodplain construction phase of the geomorphic cycle on the Fremont River. The flood was followed by a season of high base flow, and several hydrographically quiet years. Cottonwood has not regenerated downstream from the confluence of Muddy Creek at Hanksville, where the spring flood becomes insignificant compared to late season floods. The ten storage reservoirs in the Fremont basin are high in the watershed, and are operated for irrigation storage only. Their passive regulation of the flow of the lower river most likely aided the 1973 cottonwood regeneration.