bayous, and rivers — all of these types of streams vary in their size, depth, speed, and
location. Streams are always-changing natural objects where water flows downhill, taking
turns through hills and plains as elevation, rock type, and topography guide the stream
along. They are responsible for a great deal of erosion and can create great canyons over
time, as they slowly move soil, pebbles, and even boulders downstream.
Parts of a Stream
The place at which a stream originates is called thesource; this is often a spring but it
could be the top of a mountain. When two streams come together, the point where they
join is called aconfluence. The smaller of the two streams is considered atributaryof
the larger stream. Apoolin a stream is somewhat like a swimming pool — it’s a slow part
in the stream where water moves more slowly, so that the stream spreads out and becomes
deeper. Finally, the point at which a stream comes into a large body of water, like an ocean
or a lake is called themouth. These areas are calledestuaries,and they oftentimes form
unique ecosystems where water from the stream and the lake or ocean mix together (Figure
13.8).
Figure 13.8: (Left)This estuary at Damas Island, Costa Rica, shows how water, plants, and
land all come together in an estuary. (Right) This is a satellite image of the Nile Delta,
showing the unique ecosystem around the estuary, and its shape (the name delta comes from
the greek letter Δ). ( 8 )
Rivers
Rivers are the largest type of stream, and move large amounts of water through landscapes
from higher to lo through landscapes from higher to lower elevations. North America has
severaldividesthat separate the land up into separate water basins (Figure13.9). In each
of these sections, rivers will eventually run to the Atlantic Ocean, Pacific Ocean, the Great
Lakes, Arctic Ocean, or the Gulf of Mexico. Most rivers are bordered byfloodplains, which
are flat areas that flood when rivers overflow their banks.