Streams seek
equilibrium,
never find it

Variations on Aristotle's speculations guided generations of philosophers to conclude that the waters of rivers originated from a system of boundless underground fountains.

The actual source of all this water was a speculative mystery until the mid-17th century.

It was then that it occurred to scientists to make measurements to test and hopefully verify their speculations. The scientific revolution was in full bloom, following Galileo's rebellious brainchild, the scientific method. Data and controlled experiments to test hypotheses became the adjudicator of physical reality.

In France, Pierre Perreau made some measurements that changed everything.

Perreau thought that river water originates from precipitation that falls on land and is transported to the ocean by rivers. To test this, he compared his measurements of the amount of water flowing in the River Seine to his calculation of of the the amount of rainfall in the region that drained into the river.

He discovered that the water in the Seine accounted for only one-sixth of the rainfall.

The early philosophers had been asking the wrong questions. The problem was not where the water comes from, but where the rainfall goes?

As it turns out only about one-fourth of Earth's overall precipitation flows into the world's oceans in rivers. The rest seeps into the ground to become groundwater or is retained as soil moisture, stored as glacial ice, or returned to the atmosphere, either by evaporation, or by respiration of plants and animals.

It is part of the planetary hydrologic cycle between surface and atmosphere. Water carries the energy of the sun when it evaporates to mix as a vapor with the gasses of the atmosphere. It drives the weather as it condenses and releases its latent energy. The clouds that result are the source of precipitation, which is the primary factor in stream dynamics.

All channeled water flow shares certain characteristics in its dynamics and behavior, so geologists refer to any channel that water flows in as a "stream," regardless of size.

Streams are complex dynamic systems, ever-changing in response to external factors, be it precipitation, a landslide that creates a dam, or man's intervention. Every stream is always striving for equilibrium state, which it never quite reaches, at least not along its entire length, and never for very long anywhere.

There is an obvious equilibrium between input and output of water, but the stream is also seeking a balance of energy between the amount of sediment that's available, and the its ability to move that sediment.

Through this dynamic equilibrium, streams are important geologically in shaping the landscape through erosion, transportation, and deposition of sediments.

Early speculators about the origin of the landscape thought that streams simply flowed in valleys that were created by some unknown process.

Leonardo da Vinci, in the 14th century, first suggested that streams actually cut their own valleys.

Today, we understand that streams cut their own valleys. They erode downward, sideways, and upstream, and meander across their own stored sediments on a floodplain. In the process, they lengthen their courses, decrease their gradients, and widen their valleys.

Precipitation is not the only unpredictability that the equilibrium-seeking stream has to deal with. Gravity-driven mass movements such as landslides play an important role in widening valleys by moving material into the stream bed where the stream can erode and transport it.

They play a role in the dynamics of a stream both by adding sediment that is available and by influencing the energy balance of the stream.

A single event such as a landslide that temporarily dams the stream can have repercussions for miles above and below the blockage as erosion and deposition take place, respectively.

Thus, a stream system includes not only the water in the stream, but the entire drainage area of the stream as well as sediments that in transit in the stream or are stored temporarily on the floodplain.

We often forget this when trying to live with rivers because we focus our attention on the amount of water in the river at any given time. We build cities on floodplains and then are surprised when the flood comes. The floodplain is merely sediment that it is not using at this moment in geologic time, and is as much a part of the river as the water.

Rivers are continuously reshaping the landscape. They erode land that we want and deposit sediment in places where we don't want it, but the small-scale changes that take place in human time are insignificant in geologic time.

One of the most important factors influencing the geologic impact of a river is the velocity of its water. A swiftly flowing river erodes and transports more sediment and larger particles than a slow river.

The gradient of the river is the main control on velocity, but the shape and roughness of the channel are also factors. The wide, flat channel of a shallow, rocky river tends to slow the stream. A deeper stream flowing over muds and clays will offer less resistance, and it will flow a little faster.

The velocity of a river also depends upon the amount of water in the stream channel, and this can vary immensely because it is determined by weather in the form of precipitation in various portions of its drainage area.

The basis of stream dynamics is a state of balance between erosion, transportation and deposition. It is this hopeless quest for equilibrium that drives streams to reduce craggy mountains to rolling hills and ultimately to flat plains.

A stream will make adjustments to try to maintain a state of equilibrium between the load of sediment that it is carrying and its ability to carry it.

If the load decreases, the stream has excess energy and it will erode its bed or banks. If the load increases or either the velocity or amount of water decreases, the stream will not be able to handle the load, and so will deposit some of it somewhere along its course, depositing the bigger particles first and carrying the smaller particles favorably downstream.

Unfortunately, our plans are not always in harmony with the goals and objectives of the stream. As streams adjust to the capricious whims of the weather, they interfere with our lives and activities.

Or is it the other way around?