California defines Class III streams as those where no aquatic life exists and having the potential to cause sediment problems in Class I and II fish bearing streams. Current law requires that landowners exclude equipment and in some cases modify their harvest practices in these areas.
Scientific research has shown that Class III stream buffers, which are usually smaller but collectively cover a much larger area than Class I and II streams, can contribute important downstream water quality benefits. There are three reasons for this. First, Class III buffers provide a level of temperature control beyond their immediate boundaries. This is because cooler air and water that is created by the shading from Class III buffers will flow down to Class I and II streams thus creating cooler water temperatures in those fish-bearing streams.
Second, restricting harvest along Class III waters will reduce the risk of sediment delivery that is released downstream. Class III streams can be a significant sediment source because they make up the many small tributaries that are often in steep headwaters and hollows. Increasing vegetative cover along these tributaries reduces the risk of sediment delivery and over the long-term should help improve water quality and fisheries values.
The third reason relates to debris torrents, which commonly follow Class III channels. An overstory canopy won’t stop a debris torrent, but it will ensure that there are sufficient large logs mixed in with the sediment to mitigate the damage and make it more similar to a natural process.