Between the two of them, the Sacramento and the San Joaquin Rivers drain most of the state of California, collecting and concentrating rainfall and snowmelt form the Sierra Nevada and funneling it toward San Francisco Bay. On the western side of the great Central Valley in northern California, the two rivers flow together into an inverted delta before heading westward into the bay. An inverted delta is the name geologists give to river deltas in which the braiding and branching out of the river channel occurs inland, before the river actually reaches the large body of water toward which it flows.
This Landsat image shows the joining of the Sacramento and San Joaquin Rivers just north of the city of Antioch (bottom left). Near their confluence, the Sacramento and San Joaquin Rivers fan out and drop their sediment in the Central Valley before rejoining and flowing out toward the Pacific Ocean. This inland sediment-delivery system is one reason why the soils are so fertile and why farming is so productive in the valley. Pumping stations and river-sized canals deliver Sacramento Delta water to farms and cities across the Central Valley. The lush green agricultural land dominates the eastern side of the image.
Although the pumping allows productive agriculture in places of California where rainfall and natural stream flow wouldn't sustain such high-yield farming, the pumping may be having a downside. Scientists are becoming alarmed about the state of a range of fish populations in the Sacramento Delta. According to a report from National Public Radio in late November 2005, populations of several species of fish in the delta hit record lows in the previous three years. The decline has been rapid and surprising to many scientists, and although several explanations are being considered—toxic run-off, drought, invasive species—increasing delivery of water from the delta to agricultural areas farther south during the winter months appears to be the most likely culprit.
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.