The effects of hydrologic manipulation on riparian area functioning have been particularly well documented along the middle Rio Grande (Shaw and Finch, 1996; Molles et al., 1998). Historically, the middle Rio Grande was a flood-dominated ecosystem. Spring snowmelt from the mountains of southern Colorado and northern New Mexico produced peak discharges between mid-May and mid-June, based on analysis of more than 100 years of flow records prior to impoundment (Slack et al., 1993). As in other floodplain systems, overbank flooding was an integral component controlling the structure of the riparian forest.
Given the relatively frequent flooding of the middle Rio Grande floodplain systems, the riparian area was a complex mosaic of vegetation types, including cottonwood (Populous deltoides ssp. wislizenii), Goodding willow (Salex gooddingii), wet meadows, marshes, and ponds. However, dam construction in the upper basins, river channelization, and water management policies of the twentieth century have cumulatively prevented annual spring flooding in recent decades. For the middle Rio Grande, the last major floods in which large-scale cottonwood establishment occurred were in the spring of 1941 and 1942. Thus, most of the current cottonwood gallery forest reflects a legacy of flooding that occurred over half a century ago.
Structural changes in the riparian vegetation have been rapid and well documented. For example, half of the wetlands in the middle Rio Grande have been lost in just 50 years (Crawford et al., 1993). Cottonwood germination, which requires scoured sandbars and adequate moisture from high river flows, has declined substantially (Howe and Knopf, 1991). Meanwhile, invasion by exotic phreatophytic plants such as saltcedar and Russian olive has greatly altered the species composition of the riparian forests within the valley. Native cottonwood stands are in decline in many sections of the river, and the cottonwood-dominated bosque at the Nature Center in Albuquerque has experienced a 40 percent decline in cottonwood leaf litterfall over the past decade (see figure below). Without a change in water management strategies, exotic species are predicted to dominate riparian forests within the next 50–100 years.