Small streams can be the spawning factories of a watershed's wild brook trout populations. Adult trout tend to ascend them in the fall, spawn, and then leave very soon thereafter, often spending only a few days in the stream, leaving the eggs and young to fend for themselves. Not all adult trout come from elsewhere; some live their entire life in a very small stream, and it's quite a sight to see a 9 inch trout in a stream two feet wide. Brook trout need habitat that offers them relatively slow water, food and protection from predators (mink are particularly good at catching trout).
Streamside (or riparian) vegetation, especially trees, provide much of the needed shading to a stream, which helps cool a stream. Certain stream fish species such as brook trout need cool water in the summer, and the removal of riparian trees typically leads to higher summer water temperatures. Those riparian trees also provide stability to the stream banks and floodplain, and help reduce erosion that can smother fish eggs and habitat.
One of the most important habitat elements in streams is instream wood, which originates as trees in the streamside area. When a tree dies and falls into a stream, it begins the long process of providing habitat to the stream's inhabitants, such as cover where fish can hide. But it also does something indirectly - it can help form pools, where there is protection from fast-flowing water. Additionally, instream wood can help catch sticks and leaves, and these wood/leaf jams are the hotspots of microbial activity in a stream. Those microbes (bacteria and fungi) are able to absorb nutrients such as nitrogen and phosphorous directly from the water. They are eaten by shredder type aquatic insects that literally shred the leaves to eat them. Most of the nutrients those insects get from this shredding are from the microbes, not the leaves. Fish rely on instream insects for much of their diet, but particularly so in the spring, when they often do most of their growing for the year.
It can take 200+ years for many tree species in the northeast to live, die and then fall into a stream. Research has shown that the streams with the healthiest levels of instream wood are found where the dominant riparian trees are 300+ years of age. I like to say that "more wood = more trout" and that has been documented for brook trout in New Hampshire and elsewhere. Our streams and fish deserve to be healthy, and one of the best ways to achieve that is to allow some riparian trees to get big and die naturally.
To learn more about NH Fish and Game's Fish Habitat Program click here.