Page 316 - Laboratory Manual in Physical Geology

Stream Processes and Landscapes develops over rocks that are fractured or faulted
in two main directions that are perpendicular
Recall the last time you experienced a drenching rainstorm. (at nearly right angles) and break the bedrock
Where did all the water go? During drenching rainstorms, into rectangular or square blocks. The streams
some of the water seeps slowly into the ground. But most erode channels along the perpendicular fractures
of the water flows over the ground before it can seep and faults.
in. It flows over fields, streets, and sidewalks as sheets
of water several millimeters or centimeters deep. This is ■ Radial pattern —channel flow outward from a
called sheet flow. central area, resembling the spokes of a wheel. Water
Sheet flow moves downslope in response to the drains from the inside of the pattern, where the
pull of gravity, so the sheets of water flow from streets “spokes” nearly meet, to the outside of the pattern
and sidewalks to ditches and street gutters. There, (where the “spokes” are farthest apart). This pattern
the water is channelized. It flows into small stream develops on conical hills, such as volcanoes and some
channels that eventually merge to form larger streams. structural domes.
Larger streams merge to form rivers, and rivers flow ■ Centripetal pattern —channels converge on a central
into and outlet waterbody (lake or ocean). This entire point, often a lake or playa (dry lake bed), at the
drainage network, from the smallest upland tributar- center of a closed basin (a basin from which surface
ies to larger streams, to the largest river ( main stream water cannot drain because there is no outlet valley).
or main river ), is called a stream drainage system
( FIGURE 11.1A ). ■ Annular pattern —a set of incomplete, concentric
rings of streams connected by short radial channels.
Stream Channel Types and Their Sinuosity This pattern commonly develops on eroding structural
domes and folds that contain alternating folded layers
Stream channels have different forms when viewed
from above (map view). They may be straight, of resistant and nonresistant rock types.
sinuous, or meandering, and they may become braided ■ Trellis pattern —resembles a vine or climbing rose
( FIGURE 11.1B ). Sinuosity is a measure of how much a bush growing on a trellis, where the main stream
stream meanders side-to-side, the way a snake crawls. is long and intersected at nearly right angles by its
It can be calculated by dividing the length of a stream tributaries. This pattern commonly develops where
channel (along the winding path of water flow) by the alternating layers of resistant and nonresistant rocks
straight-line distance from start to end of the stream have been tilted and eroded to form a series of parallel
segment ( FIGURE 11.1 ). To measure the length of a ridges and valleys. The main stream channel cuts

stream channel, lay a string along the stream channel through the ridges, and the main tributaries flow
on the map (along the winding path of water flow), perpendicular to the main stream and along the valleys
then stretch out the string along the bar scale to find (parallel to and between the ridges).
the length. Perfectly straight channels have a sinuosity
of 1, but streams in this lab are classified as straight if ■ Deranged pattern —a random pattern of stream chan-
their sinuosity is less than 1.3. Sinuous streams have nels that seem to have no relationship to underlying
sinuosities from 1.3 to 1.5. Meandering streams have rock types or geologic structures.
sinuosities greater than 1.5.
Drainage Basins and Divides
Stream Drainage Patterns The entire area of land that is drained by one stream, or an
A stream drainage pattern is the arrangement of entire stream drainage system, is called a drainage basin .
stream channels and tributaries that forms on a The linear boundaries that separate one drainage basin
landscape as a result of its underlying geology and relief. from another are called divides .
These are some common stream drainage patterns Some divides are easy to recognize on maps as

( FIGURE 11.2 ): knife-edge ridge crests ( FIGURE 11.3 ). However, in

regions of lower relief or rolling hills, the divides
■ Dendritic pattern —resembles the branching of a separate one gentle slope from another and are more
tree. Water flow is from the branch-like tributaries to difficult to locate precisely ( FIGURE 11.1A , dashed line

the trunk-like main stream or river. This pattern is surrounding the Tributary X drainage basin). For this
common where a stream cuts into flat lying layers of reason, divides cannot always be mapped as distinct
rock or sediment. It also develops where a stream cuts lines. In the absence of detailed elevation data, they
into homogeneous rock (crystalline igneous rock) or must be represented by dashed lines that signify their
sediment (sand). most probable locations.
■ Rectangular pattern —a network of channels with You may have heard of something called a continental
right-angle bends that form a pattern of intercon- divide, which is a narrow strip of land dividing surface
nected rectangles and squares. This pattern often waters that drain in opposite directions across the

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