University of Colorado GEOLOGY 1010 
Class Note 13

Streams

A stream is a body of water that is confined in a channel and moves downhill under the influence of gravity. This definition includes all sizes of running water from a tiny trickle to the Amazon.

The total area drained by a given stream is its drainage basin.

The line separating one drainage basin from another is a drainage divide. The Continental Divide separates drainage basins of rivers that flow to the Atlantic and Gulf of Mexico from those flowing to the Pacific and Gulf of Cortez (Gulf of Claifornia).

Normally less that 20% of rainfall runs off in surface drainage while the remaining 80% soaks in to the ground or evaporates. During times of abnormally high rainfall runoff can approach 100% of rainfall.

Floods can be flash floods as occurred in the Big Thompson Canyon in 1976, or annual floods due to snow melt in the higher elevations of a drainage basin.

Stream velocity is governed by gradient. Boulder Creek falls from Nederland at about 8000 ft to Boulder at 5600 ft over a distance a distance of 15 mi and thus has a gradient of 160 ft/mi. The Colorado River through Grand Canyon has a gradient of less than 10ft/mi. The lower Mississippi has a gradient of 0.5 ft/mi. Stream gradient usually decreases down stream.

Stream velocities vary with gradient and channel size. The lower Mississippi typically moves at less than 1 mi/hr. Channel constrictions cause an increase in stream velocity.

Although velocities and gradients may vary along a given channel with no tributaries or dams, the discharge rate is constant. The discharge rate is the amount of water passing a given point in unit time. In the U.S.A. (and nowhere else) we measure discharge in cubic feet per second (cfs). The rest of the world talks about m3/s. (1m3 = 35 ft3)

Boulder Creek is typically about 50cfs through much of Fall and Winter, but may approach 1000 cfs during late May and early June. The Colorado at Grand Junction is typically 2000 cfs during fall and winter but may exceed 20,000 in May and June. The Colorado above Lake Powell and below the Green (Cataract Canyon) is typically 5000 cfs but may peak above 100,000 cfs in flood. Typical discharges from Glen Canyon Dam are 8000 to 12000 cfs. The Mississippi in the 1993 Flood peaked at over 3 million cfs. Current stream flows in Colorado are given here.

The total amount of material transported by a stream is its load. A stream can transport solid material as dissolved load, bed load, or suspended load. The bed load may move by saltation (lifting of grains off the bottom or traction (rolling rocks along the bottom).

The amount of bed load and suspended load vary with velocity (not discharge rate).

Where a stream slows and broadens, its load decreases, and it deposits material. Where it narrows and quickens, it erodes material. Sand and gravel deposited in a stream channel (side or middle) is a bar. A bar at the side of a channel is a point bar. If many bars are A bar in the midfdle of a channel is a channel bar. If many bars are deposited the stream channel may split and rejoin many times. Such channels are said to be braided. Here is an interesting study on beaches on the Colorado in Grand Canyon.

Meanders develop where a stream curves and the outer part of the channel moves fast er and erodes material (called a cut bank) while the inner part of the curve moves more slowly and deposits material (called a point bar). If the river intersects itself it can cutoff the meander and form an oxbow lake. Meanders are normally ephemeral, constantly changing. They move up or down a flood plain. However if a stream such as the San Juan or the Colorado develops a mature channel with meanders, and then the drainage basin is uplifted, the meanders can become incised and fixed.

 Here, meanders on the Green River in Canyonlands National Park in Eastern Utah have incised or entrenched themselves.

When a river flows into standing water, it drops its load and forms a delta. Such streams are depositional and the bars formed split the channel into distributaries


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