runoff. In the absence of significant tectonic disturbance, the overall effect over
very long amounts of time is to make the higher places lower by erosion and lower
places higher thus “smoothing” the landscape. Within the immense timescale
related to smoothing it is common for streams to produce hills, valleys, and can-
yons by lengthening, deepening, and widening their valleys
The landscape has been given a simple classification vis-a`-vis stream erosion. The
surface area drained by a stream is a drainage basin (or a watershed) and itsbounda-
riesare drainage divides. Drainage basins exist on vastly different scales and,
because streams have tributaries, main streams are associated with the multitude of
tributary drainage basins that are nested at various scales one within another. It is
through the organization of drainage basins that the flows of mass and energy asso-
ciated with stream erosion and deposition can be understood. Stream-produced land-
scapes are segmented into interfluves, valley sides, and valley bottoms. High ground
dividing adjacent river valleys is known as an interfluve. At the highest elevation
along an interfluve is a drainage divide. Interfluves can be many kilometers across
or nonexistent in the case of drainage basins edged by steep mountains. The valley
side is the increased slope down to the valley bottom. Valley sides can have slopes
of only a few degrees from horizontal or be bluffs or canyon sides. The valley bottom
is the surface along which a stream is eroding or depositing and can be negligible
such as on a canyon bottom or many kilometers wide.326 Stream Erosion and Deposition
Equilibrium
The concept of equilibrium is of great consequence in geography as in other modern sciences.
Systems are explained by studying a factor (or a combination of factors) over time and describ-
ing its (their) changes. From this comes a sense of the average condition of a system that can be
conceptualized as a balance of factors. In thatEarthis an open system, mass and energy enter
and leave the Earth’s system so that external factors disturb equilibrium conditions. For
instance, the Earth’s energy balance depends on the amount of solar radiation emitted from
the sun. If this amount were to increase, Earth’stemperaturewould tend to increase. Other
perturbations of equilibrium begin within the system.Plate tectonics, for example, some-
times cause mountain building, which sets off series of changes in erosion and regional weather.
Earth’s subsystems are dominated by negative feedbacks that slow and dampen changes away
from the habitable equilibrium of current Earth conditions. In the case of increasing solar radi-
ation, there would be increasing evaporation fromoceans, possibly causing morecloudsand
greater albedo, thus minimizing the change in temperature. The most fruitful geographic appli-
cation of equilibrium theory has been ingeomorphologyin the explanation of landforms. In
this instance, equilibrium theory holds that slope form is shaped by a balance of processes such
that an energy balance is maintained over the long term.