Fresh water is essential to all life on earth, and forests are essential to fresh water. Forests filter and clean water, soften heavy rainfalls which would otherwise erode soils, and hold river banks in place. In turn, water carries dissolved nutrients, which it distributes through the forest soil. Forests are "sponges", capable of collecting and storing large amounts of rainfall. Forest soils absorb four times as much rainfall as soils covered by grass, and eighteen times more rainfall than bare ground.

Water drops collect on leaves. Photo by Maya Walters

Forest canopies intercept much of the precipitation that falls over them. This rain and snow would otherwise fall directly to the ground, gradually washing soil away. The forest prevents this erosion from taking place. The water that lands in the forest canopy gently drips to the ground with much less force than direct rainfall.

The forest canopy intercepts snow before it piles up on the ground. Photo by Maya Walters

Riparian zones are the wetlands in the forest, or the areas around streams, rivers, and ponds. The water connects the forest, carrying dissolved nutrients from one area to another, as well as washing plants' seeds downstream and providing a migration corridor for animals. These areas usually support the most diverse life in the forest. find out more...

Forest cover around rivers prevents excessive erosion and flooding. In some cases, however, especially in the tropics, seasonal flooding in the forest is a completely normal and essential event. These floods can be enormous, spreading many kilometers in both directions from the normal river banks. They have occurred every year, for thousands of years, and the forest and the animals that live there have evolved to take advantage of them.

Above: A creek in a temperate forest. Note the fallen log and variety of vegetation. Photo by Maya Walters. But in most cases, floods do not occur in a predictable pattern. Logging operations in watersheds can create floods, because when the trees are removed, mountain slopes are exposed to the sunlight. Spring snowmelt then occurs much more rapidly, and the excess water fills streams too quickly, and flooding is the result.

In general, rainfall is heaviest around the equator. The farther away from the equator, the longer the dry season. Since water is so important for maintaining the forest's canopy of green leaves, areas with longer dry seasons tend to have deciduous* forests. In these places, trees lose their leaves during all or part of the dry season. In temperate forests, trees lose their leaves as the cold weather begins, but even tropical forests can be deciduous, sometimes only losing their leaves for a few weeks while water is in shortest supply, and regrowing them as the rainy season begins.

Water and wet soil carry dissolved nutrients gradually downslope. If a forest is fragmented, however, by a road or structure, this nutrient dispersal is intercepted. Instead of being evenly distributed throughout the soil, water pools up behind the obstruction or runs along it, and few nutrients from higher up reach the trees downslope from the interruption. Water eventually finds its way to valley bottoms, where it collects in riparian* areas. Water drops collected on a leaf. Photo by Maya Walters.

In addition to carrying nutrients, water will also carry toxins and waste materials downstream. Water connects the forest, and anything that pollutes the water pollutes the forest. High alpine streams might not contain any fish, but hundreds of these small threads of water run downslope into larger creeks, which run into larger rivers.

Waterdrops on a lupine leaf. Photo by Maya Walters.

Pollutants in any of these small streams find their way into the rivers where they collect and damage fish habitat. Effects from any disturbances along forest streams ultimately affect all habitats between their source and the ocean.