Driven by a need for fresh produce and drinking water, the United States
now utilizes as much as 90 percent of the water that flows into the
Colorado River. As a result, very little of the Colorado River now
reaches the Colorado River Delta. Many environmentalists and scientists
alike fear the natural ecosystem in the delta may be beyond repair.
Yet, no one really knows what the ecosystem in the delta was like before
its initial decline in the early 20th century. In order to unravel the
delta's past, one group of researchers at the University of Arizona has
been using images much like the one above to locate and study old clam
shells in the delta. By examining these shells, the researchers believe
they can determine how abundant marine life was around the Colorado
River Delta when the waters of the Colorado River flowed freely.
The above image of the lower eastern section of the Colorado River Delta
was taken on September 8, 2000, by the Advanced Spaceborne Thermal
Emission and Reflection Radiometer (ASTER) flying aboard NASA's Terra
spacecraft. The Colorado River comes to an end just north and east of
this image in Baja, California. The far northwestern shore of the Gulf
of California can be seen in blue on the right hand side of the image.
The gray expanse bordering the gulf are mud flats formed by sediments
deposited over hundreds of years by the Colorado River. The large white
patches to the left of the mud flats are highly reflective salt pans.
Piles of dead shells accumulate along the southeastern portion of the
mud flats. They make up ivory-colored beaches and mostly lie along the
border between the gulf and the mud flats.
Karl Flessa, a paleontologist at the University of Arizona, explains
that the appearance of these clamshell "islands" themselves is due to
the damming and draining of the Colorado River. Most of the sediments
that the Colorado River picks up as it moves through the American
Southwest are now captured behind the Hoover Dam or the Glen Canyon Dam
well before they reach the river delta. Without a steady supply of
sediments, the mud flats have been slowly eaten away by the waters of
the Gulf of California. Each day as the tide moves back and forth
across the flats, the waters remove small sediments and expose old
Flessa and his group are studying the clamshell "islands" to learn more
about what the environment was like when the river flowed into the gulf.
They employ Landsat 7 images much like the image shown above to locate
old clam beds and find their way around the mudflats. Once there, Flessa
explains he and his team scoop up large samples of shells and take them
to a lab. By then isolating certain radioisotopes found in the shells,
the scientists can determine how abundant clam shells were in the past
and compare them to present day populations.
What the scientists have uncovered so far is that over the past century
the number of living clams in the delta has decreased by 95 percent.
Flessa explains that the reduction of clams is due to the increased
salinity and lower nutrients in the delta region. Most of the clams
that lived in the delta thrived in a salt water/fresh water mix. When
the river stopped flowing into the gulf, the waters became briny and the
clams died. Since the clams were near the bottom of the food chain,
it's likely that a large portion of the larger animals in the region
vanished as well.
The Colorado River is the largest watershed in the southwestern US, emptying into the Salton Trough before reaching the Sea of Cortez. Over the past 2-3 million years, river sediments built a delta that extends from the US-Mexico border for a distance of 87 miles (140 kilometers). However, today the Colorado River delta is undergoing significant erosion and diminishing in size due to the lack of sediment replenishment from upstream sources.
The Ebro River Delta, located along the eastern coast of Spain, is one of the largest wetland areas (320 km²) in the western Mediterranean region. The Ebro delta has grown rapidly—the historical rate of growth of the delta is demonstrated by the city of Amposta. This city was a seaport in the 4th Century, and is now located well inland from the current Ebro river mouth. The rounded form of the delta attests to the balance between sediment deposition by the Ebro River and removal of this material by wave erosion. This astronaut photograph, taken in partial sunglint, also shows the Ebro’s fresh water lens—the water density boundary between the upper layer of fresh water issuing from the Ebro River mouth and the saltier, denser Mediterranean Sea water.