Earth Science
Chapter 11
Section 2
A. River Systems:
Tributaries are the smaller streams and rivers that feed into a
main river.
A river and all its tributaries together make up a river system.
All the water in a river system drains into the main river.
The land that supplies water to a river system is called a
watershed.
A ridge of land called a divide separates watersheds from another.
Streams on each side of the divide flow in different directions.
The continental divide is the longest divide in North America and
follows the Rocky Mountains.
West of the continental divide, water runs toward the Pacific
Ocean or into the Great Basin.
Between the Rocky Mountains and the Appalachians, water flows
toward the Mississippi River or directly into the Gulf of Mexico.
B. Rivers and Floods:
A flood occurs when the volume of water in a river increases so
much that the river overflows its channel.
Floods are both feared and welcomed.
Floods are welcomed by people that depend on the floodwaters
leaving behind fertile soils in desert areas for growing crops (Nile)
Feared by those who live in the flood plains of China, Bangladesh
and India, and place in the U.S.
Can flooding can be controlled?
Rivers build a natural defense against flooding by depositing heavy
sediments alongside the channel. Over time these sediments build
up into long ridges called levees.
Man building up levees sometimes backfires. Levees reinforced
by man prevents the natural widening process of a river and forces
the water to go only downstream.
C. Bodies of Fresh Water:
What is the difference between a pond and a lake?
There is no definite rule, but a pond is usually smaller and
shallower than a lake.
Ponds and lakes form when water collects in hollows and lowlying areas of land.
Ponds – A thriving habitat that supports a wide variety of living
things. The bottom of a pond is usually covered with mud and
algae.
Lakes - Lakes form in different ways. A meander in a river can
be cut off forming an oxbow lake. A hollow in the ground can be
filled with ice from an ice age. The crater of a volcano collapses
forming a caldera and fills with water. Some lakes are man made,
such as Lake Mead. This type of lake is known as a reservoir.
The habitat is similar to that of a pond in the shallow areas and
fewer organisms in the dark, cool deep areas where bony fish like
Pike and Sturgeon dwell.
D. Changes in a Lake:
Due to temperature the density of the water causes water to mix.
This mixing moves minerals, plant matter, and other nutrients from
the lake bottom to the surface.
Called Lake Turnover, this seasonal change refreshes the supply of
nutrients throughout the lake.
A second change in a lake occurs over time. Over many years,
nutrients build up in the lake in a process called eutrophication.
As eutrophication causes more algae to grow, a thick scum forms
on the surface of the water.
E. Icebergs:
Large masses of ice made of fresh water that have broken off from
a glacier and floated off into the ocean are known as an iceberg.
About 90% of an iceberg lies below the surface. The underwater
part is a hazard to ships because it is often much wider than the
visible part of the iceberg.