Headwater streams typically have a steep
gradient, with riffles, rapids, and falls. In
forested areas, overhanging trees shade the
channel, preventing the growth of many aquatic
plants and minimizing the role of
photosynthesis. Instead, most energy and
nutrients enter the stream in the form of
leaves and woody material (detritus).
This detritus is colonized by fungi and
bacteria. Immature aquatic insects of various
kinds, known as shredders, eat the fungi
and bacteria. Common shredders in small streams
are stonefly nymphs and cranefly larvae.