WSP11716

• File Number: 8507
• Description: Rio Grande Project
• State: CO
• Basin: Rio Grande
• Date: 7/1/1997
• Title: Water Management Study: Upper Rio Grande Basin part 1
• Water Supply Pro - Doc Type: Report/Study

Water Management Study: Upper Rio Grande Basin

character and irrigation withdrawals often leave the river bed dry. Similar
changes have occurred in Texas, and in some places the river has been
transformed into a concrete-lined ditch.

The most intensive investigations of riverine and riparian ecosystems have
occurred in the Middle Rio Grande Valley, and particularly with respect to
the cottonwood-dominated riparian forest known as the bosque. The
following discussion is drawn from a recent report of the bosque's historical
and current ecological conditions (Crawford et al. 1996) illustrating the
extent ofthe ecological changes that have occurred and the associated
problems that exist in the Middle Rio Grande Valley. The report concludes
that human manipulation of the ecosystem, especially in the past 60 years, is
having such a dramatic impact that "in terms of its structure and
functioning, [it] will undergo irreversible change in the absence of a new
management paradigm." Although one must employ extreme caution before
extrapolating from this area to others within the Basin, this general
description is indicative of ecosystem conditions, changes, and problems
elsewhere in the Basin.

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The bosque of the Middle Rio Grande V alley is the longest continuous stretch
of cottonwood forest in the American Southwest. Historically, the bosque
was distributed throughout a broad floodplain, reflecting the vagaries of
climate and the periodic flooding and drought that caused the river to shift
channel frequently. Early human settlement by Indians and Spanish
settlers had cleared some of the forest and altered the river's hydrology
somewhat, but the bosque retained most of its widespread distribution and
connection to natural hydrological patterns. Trapping of beaver between
1820 and 1840, however, resulted in the elimination of beaver dams, causing
significant increases in runoff intensity and sedimentation. The arrival of
the railroads in the 1870s stimulated greater mining, grazing, and other
activities that further altered the river's hydrology and changed the Basin's
vegetation patterns. With rapid development of irrigated agriculture in the
San Luis Valley of Colorado, downstream flows dwindled and the river began
to deposit sediment much sooner, causing aggradation (raising) ofthe river
bed. As the river bed rose, so too did the shallow water table in the
floodplain and, eventually, some land that otherwise would have been dry
became waterlogged. Other areas, however, dried out as the aggradation,
plus human intervention, left them inaccessible to the shifting river channel.
These hydrological changes, together with continued clearing of land within
the Middle Rio Grande Valley, greatly reduced the abundance of the bosque's
cottonwoods.

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