Until the 1930s, flows of the Colorado River maintained approximately
781,060 hectares of wetlands in its delta. These wetlands provided important feeding
and nesting grounds for resident and migratory birds as well as spawning and
protection habitat for many fish and other invertebrate species. However, the Delta's
wetlands started to disappear as water was used for agricultural and urban uses in the
United States and Mexico. The 1944 United States-Mexico water treaty, which
allocates 1.8 million m��/year to Mexico, did not define a minimum flow to maintain
the Delta's ecosystems. The resulting degraded Delta lead to the perception in the
1980s that the Delta was a dead ecosystem.
This study investigates whether this "dead Delta" perception is valid. Its
central hypothesis is that regenerated vegetation in riparian and flood plain zones is
associated with surplus river flows during the 1990s. A vegetation analysis, using
satellite imagery and field methods, shows that native trees have regenerated during
the last 20 years, and now account for 23% of vegetation in a 100 km, non-perennial,
stretch of river below the United States-Mexico border. A spatial trend analysis using
multi-temporal data on percent vegetation cover indicates that there are 6,320 hectares
that show a significant increasing trend (p-value<0.05) in vegetation cover, with the
Delta's riparian zone having at least 18% of its area showing this trend.
The study estimates that once in four years February to April flow of 300
million m�� (at 80-120 m��/s) is sufficient to germinate and establish new cohorts of
native trees, and highlights the need for smaller but more periodic flows in order to
maintain wetland areas. It is concluded that there is clear evidence of the resilience of
the Delta's ecosystems and that the "dead Delta" perception is no longer valid. There
exist critical habitat in the Delta that needs to be protected, while there also exist short
and long term opportunities to ecologically enhance and expand current habitat.
Hydrological and ecological studies are needed to estimate specific water
requirements for these areas in order to efficiently target them for immediate and long
term conservation actions. / Graduation date: 2003
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31139 |
| Date | 07 October 2002 |
| Creators | Zamora-Arroyo, Jose Francisco |
| Contributors | Pease, James R |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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