Running Water - The Changing Earth: Exploring Geology and Evolution - page 318

Range of western North America have radial drainage
patterns.
In all of the types of drainage mentioned so far, some
kind of pattern is easily recognized. Deranged drainage , in
contrast, is characterized by irregularity, with streams fl ow-
ing into and out of swamps and lakes, streams with only
a few short tributaries, and vast swampy areas between
channels (Figure 12.19e). This kind of drainage developed
recently and has not yet formed a fully organized drainage
system. In parts of Minnesota, Wisconsin, and Michigan,
where glaciers obliterated the previous drainage, only 10,000
years have elapsed since the glaciers melted. As a result,
drainage systems have not fully developed and large areas
remain undrained.

because they must have some gradient to maintain fl ow.
So ultimate base level is sea level, which is simply the low-
est level of erosion for any waterway that fl ows into the sea
(

Figure 12.20). Ultimate base level applies to an entire
stream or river system, but channels may also have a lo-
cal or temporary base level . For example, a local base level
may be a lake or another stream, or where a stream or river
flows across particularly resistant rocks and a waterfall
develops: (Figure 12.20).
Ultimate base level is sea level, but suppose that sea level
dropped or rose with respect to the land, or suppose that the
land rose or subsided? In these cases, base level would change
and bring about changes in stream and river systems. During
the Pleistocene Epoch (Ice Age), sea level was about 130 m
lower than it is now, and streams adjusted by eroding deeper
valleys and extending well out onto the continental shelves.
Rising sea level at the end of the Ice Age accounted for a
rising base level, decreased stream gradients, and deposition
within channels.
Natural changes, such as fl uctuations in sea level during
the Pleistocene, alter the dynamics of rivers and streams, but

◗

THE SIGNIFICANCE OF BASE LEVEL
Base level is the lowest limit to which a stream or river can
erode. With the exception of streams that fl ow into closed
depressions in deserts, all others are restricted ultimately
to sea level. That is, they can erode no lower than sea level

◗

Figure 12.18 Drainage Basins

WISCONSIN

LAKE
MICHIGAN

MICHIGAN

INDIANA

ILLINOIS

Small drainage basins are separated by divides (dashed lines),
which are along the crests of ridges between channels (solid lines).

Fort
Wayne

a

OHIO

INDIANAPOLIS

MISSOURI
PLATTE

UPPER
MISSISSIPPI

Terre
Haute

Bloomington

OHIO

ARKANSAS
RED-WHITE

WHITE

LOWER
MISSISSIPPI

KENTUCKY

Gulf of Mexico

A detailed view of the Wabash River's drainage basin, a tributary
of the Ohio River. All tributary streams within the drainage basin,
such as the Vermilion River, have their own smaller drainage basins.
Divides are shown by red lines.

c

The drainage basin of the Mississippi River and its main
tributaries.

b