A stable structure readily adjusts to changing forces, resisting push from one direction with an equal push from an opposing direction. The structure remains in equilibrium, a balanced state, until the load changes again.

Seasonal flooding is just one of many factors that causes the forces acting on a dam to change. A dam in good repair can adjust to the changes accordingly. When structural defects such as cracks in its wall become apparent, however, a dam must be either repaired or removed. Doing nothing can lead to seepage, overtopping (water spilling over the top), or even catastrophic structural failure.

There are several types of dams. Gravity dams, which include embankment dams and concrete dams, rely on their enormous mass to resist the water's horizontal thrust. The downward force provided by their weight and the upward force provided by the ground squeeze the building material so tightly that the load will not topple the dam or move it downstream. These dams are strong and relatively simple in design but require a great deal of material and can therefore be expensive.

Designing thinner-walled but equally strong arch dams instead can save on materials and overall cost. An arch dam uses the properties of the arch to channel compressive forces along its curve to supports or solid rock walls on either side. Another cost-saving design is the buttress dam, a largely hollow dam that utilizes concrete braces on its downstream side for additional mass and greater support.

Assessing a dam's "fitness," however, extends beyond its physical well-being. Dams dramatically alter ecosystems and can affect human lives as well. They might disrupt the migratory paths salmon use to get to their breeding grounds, for example. And more than once, the upstream flooding that occurs when a dam is built has forced inhabitants of entire villages and cities to relocate. Sometimes, it turns out, the best choice is to have no dam at all.