CHAPTER 9 - HYDROLOGIC CYCLE

The "Hydrologic Cycle" refers to all processes involving the movement and accumulation of water (and water vapor and ice) through the atmosphere, on the Earth's surface, and beneath the surface.

The "Water budget" - the total amount of water on earth is about 1.36 Billion cubic kilometers; covers ~71% of earth's surface.

HYDROLOGIC CYCLE

EVAPORATION - molecular exchange of a liquid and a gas (different than boiling); water evaporates off of wet surface.

TRANSPIRATION - moisture given off by plants (by evaporation).

PRECIPITATION - condensation and gravitational fall in the form of rain, ice, and snow.

INFILTRATION - water soaks into the ground.

RUNOFF - unable to soak in and flows on surface (cause erosion).

STREAMS

Any water flowing on the surface is considered a "stream" (including rivers, streams, ditch, gully, brooks, creeks, arroyos, rios, or whatever term is applied). Streams are fed by runoff and by spring venting groundwater to the surface. Rivers are fed mostly by groundwater flow, "floods" occurring only during periods of precipitation or melting of ice.

INTERMITTENT STREAM: any stream that flows when after a rain or during a rainy season. Part or most of the year the stream channel is dry.

Thc capacity of a stream to ERODE the landscape based on three factors:

LONGITUDINAL PROFILE OF A STREAM

These terms are used to describe characteristics of a stream drainage system.

CHARACTERISTICS OF STREAM SEDIMENT TRANSPORT

A "GRADED STREAM" has the correct gradient, channel characteristics, and discharge to maintain transport of eroded materials.

BASE LEVEL - lowest elevation to which a stream can erode.

WORK OF STREAMS

Erosive forces of moving water scour the stream channel.

POTHOLES - scoured pits carved by pebbles swirling in eddies on the stream bottom.

TRANSPORT of sediments in streams occurs three ways:

  • 1. DISSOLVED LOAD - IONS in SOLLUTION (measured in "parts per million" (PPM).

    sea water has about 35,000 ppm dissolve solids

  • 2. SUSPENDED LOAD - clay, silt, and fine sand are "carried" by turbulent, moving water when the current too strong to allow materials to settle out; in some cases there may be more sediment than the water carrying it! (A famous quote about the Missouri River is that the flooded river is "Too thick to drink; too thin to plow.")

  • 3) BED LOAD - sand, pebbles, cobbles, boulders too large to be carried is suspension. Larger particles move along the bottom of the stream by SALTATION - rolling, bouncing, and sliding.

    EFFECT OF URBANIZATION

    In the process of developing an urban landscape, certain practices regarding the control of stream drainage have resulted in catastrophe. Future generations of humans need to understand the concepts of "resource management." The problems of urbanization include:

    Urbanization increased risk of flood damage!

    EFFECT OF BUILDING A DAM

    The construction of a dam causes all kinds of disruption to the natural stream drainage processes. Dam RESEVOIRS accomodate flood waters by distributing flood discharge over a large surface area. This spreads flood discharge over a longer period of time, lowering the peak of flood stage downstream. The problems with dams include:

    STREAM SEDIMENTATION CHARACTERISTICS

    A streams ability to carry sediment depends on two criteria:

    Concepts related DEPOSITION by streams:

    STREAM CHANNEL MIGRATION

    DRAINAGE SYSTEMS AND PATTERNS

    The progress of time allows streams to modify the landscape and to change the region drained by streams. Geologic features in a region, (such as faults, rock types, and structures) control, in part, the progress of stream development. Different geologic structures result in recognizable stream drainage patterns.

    STAGES OF STREAM VALLEY DEVELOPMENT

    As streams progress in age they modify the landscape:

    Return to the Class Web Page.