Allison Gibson, age 13, of Monroe, La., for her question:
WHAT CAUSES A RIVER'S DELTA?
A delta is a land area that is built up where a stream or river ends. It in caused by earth or sand that is carried along by the water. When the water meets a see or lake, its speed is suddenly slowed and most of the sediment the water has been carrying is dropped.
Mud and other sediment is piled on the sea bottom, layer after lays:, until it finally reaches the water surface. All along the shoreline or margin of the quiet water body, the delta cannot be built higher than the water surface by the river or stream, although waves can throw up beaches and winds may very well pile loose sand into dunes on the delta surface.
Upstream, the river sometimes overflows onto the delta's surface during times of flood. At these times, mud, soil and sand can be spread on a growing delta. Thus, the delta's surface often rises.
Plants often grow on the moist delta and their roots and remains are then mixed with the stream deposits, forming rich though often wet soil.
Sometimes the muddy river or stream deposits enough sediment to fill up its original channel. When this happens, the stream overflows across the eloping delta's surface and other channels are formed, leading away from the main stream.
The delta received its name from the fourth letter of the Greek alphabet, which is shaped like a triangle. The name was first given to the lower Nile River plain many centuries ago by Greek merchants who noticed that the branching mouths of the Nile enclosed a flat triangular area with its base along the Mediterranean Sea.
A delta's shape and size is the result of the amount of earth or sand brought by the river or stream and also on the strength of the waves and shore currents in the water body. Where such currents and waves are strong, the deposits may be carried away as fast as they are brought and no delta may be formed. Light currents and waves allow the sediment to form a delta.
About 1 million tons of mud and silt is carried by the Mississippi River each day. The delta of that great river is extended about 250 feet each year, so that it grows an additional mile every 20 years.
By cutting down many forests and also by overgrazing grasslands, man has increased the rate at which sediment is washed away by rivers and thus the rate of delta growth.
Some deltas have produced agricultural regions that have benefited man. Just about the entire country of the Netherlands is in the delta areas of Europe's great Rhine River, the house, Scheldt and other rivers.
Southeastern California's Imperial Coachella Valley, a rich agricultural area, was once the head of the Gulf of California, but was cut off by the Colorado River's delta.
Delta land is frequently subject to flooding.