Riparian management zones (RMZs) function as an important part of a watershed analysis unit (WAU). It is commonly referred to as the transition zone between upland (harvestable forests) and aquatic zones (streams). The purpose of the RMZ is to provide a sufficient stream ecosystem of sustain fish habitat. The riparian function is defined by two specific processes: (1) the recruitment of large woody debris (LWD) and (2) the provision of shade to the stream. Part 1 of the process evaluates the availability of LWD in the near (10-20 years) and long (20-200+ years) term. Part 2 of the process evaluates current canopy coverage over the stream area.

Riparian Management Zone

Large Woody Debris Recruitment

The RMZ is the primary source of large woody debris. Large woody debris diverts and obstructs streamflow so it increases channel complexity. The formation of pools and backwater eddies, both of which are important for fish habitat, are strongly aided with the presence of a stable supply of large woody debris. First, we determine the dominant vegetation type. If an area contains at least 70% of one species (coniferous of hardwoods) then that species is considered dominant. If neither conifers nor hardwoods are dominant then the stand is considered mixed. Large conifers are the prime choice for the recruitment of LWD. Softwoods tend to resist decay and can be recognizable for up to a century supplying stable long term LWD. Hardwoods also contribute to LWD recruitment but only for the near term since they decay quickly, after a few decades, in wet conditions. See figures 2 and 3 for surrounding species.

LWD recruitment also relies on the size on the wood being introduced to the stream. Tree size is determined by average tree size classes in the stand. Large and medium tree sizes are considered the best for short and long term LWD recruitment. See figures 4 and 5 for species size.

Table 1 Average Tree Size Classes.

The final consideration for LWD recruitment is stand density. RMZs prefer dense stands. In Western Washington a stand is considered dense if no more than 1/3 of the ground is exposed. By using the average trees per acre in a stand and assuming that the average tree has a crown diameter of 16 feet we can find stand density. For example, a stand has 200 trees per acre and an acre has 43560 square feet. If the tree crown is 16 feet in diameter it encompasses about 200 square feet. To be considered a dense stand, trees must cover about 29000 square feet of land, if we take 200 square feet per tree and multiply it by 200 trees per acre we find that the trees cover 40000 square feet per acre. This is considered a dense stand. For figuring species density I assumed that stands over 145 trees per acre were dense. See figure 6 for stand densities.

From these maps we can locate the areas that fulfill the criteria for good riparian management zones. Figure 7 shows the areas of moderate to good riparian management zones.

Table 2 Recruitment Potential Rating (coded H or C species; S, M or L species size; S or D stand density).

The areas not coded are areas with small conifers in dense stands and have a low recruitment potential rating. This low rating reflects the low potential for near term recruitment. These areas will be upgraded to a moderate or high potential rating, as the conifers grow larger.

Canopy Closure/Stream Temperature

The amount of shade supplied to the stream is a function of tree height and sun angle. For instance a tree given a height of 50 feet, assuming a constant solar angle of 60 degrees, will provide shade to one half of a stream with a width of 58 feet. Or if a stream is 40 feet wide it will need riparian vegetation of height 35 feet for one half of the stream to be shaded. This is if the vegetation is on the stream banks. If the canopy is fairly sparse the tree will have to be taller to provide shade to the center of the stream. Average tree heights are found in figures 8 and 9.

Canopy closure effects the stream temperature. Since the lowest elevation in the Washougal watershed is about 800 feet, from pg. D-35, table 7 in the Conducting Watershed Analysis Handbook we find that the streams in the watershed have a maximum shade category of 70%. This means that only 70% shade (canopy closure) is needed to sustain fish habitat temperatures. By inspecting aerial photographs we can estimate the level of canopy closure. See figure 10

From these maps we can determine the areas that are above or below target shade levels. See figure 11 for canopy closure levels.

Table 3 Estimated Levels of Canopy Closure from Aerial Photographs.

The areas outlined in red represent the cover over the Washougal River. Since the river is wide shade levels (canopy cover) do not determine or effect the temperature of the water.

By using the steps outlined above we can identify riparian management zones that supply an adequate ecosystem to sustain fish habitat. These RMZs are vital for fish habitat and are required for all fish bearing streams. From the maps produced we can see the "high hazard areas" and can be aware of potential problems in the outlying areas.

Usually the riparian management zones are found by, not only the method discussed above, but also field verification. Field verification is important for data truthing. Actual riparian management zones may differ slightly (or greatly) depending on the relevance of the data at hand. For example, new clearcuts may have been established or trees may have grown larger changing stand densities and average species size.

Standard Methodology for Conducting Watershed Analysis (Under Chapter 222-22 WAC). WA Forest Practices, Version 4.0, November 1997.

ArcInfo, ArcView and Grid program instruction by Phil Hurvitz.

Aerial Photographs supplied by Weikko Jaross (Wa DNR).

Silviculture assistance by Dr. Jim Fridley.