A run is a section of fairly fast and fairly deep water. Depending on depth and stream bed rocks, the surface can be choppy or smooth. Runs are often faster portions of wider sections of rivers bordered on one or both sides by a wider, shallow shelf. Trout often hold at the bottom of runs or in seams between the actual run and slower adjacent water. The head, or upstream end of the run, is usually a dropoff just below a riffle. Fish will hold at the head of a run waiting for dislodged nymphs to float along or waiting for mayflies that emerged in the riffle. The end of a run is called the tail. (The word run is sometimes used by fly anglers around here to identify sections of streams and rivers–such as the Dog Run or Deer Run. These locations include several types of water.)