In addition to the bank stabilization provided by good root systems, both the amount and types of vegetation present in the riparian area influence other functions of a riparian system. Riparian vegetation protects the soil from erosion and compaction, traps sediments and nutrients, and reduces water and soil temperature and evaporation (Figure 2-19).

One common way to estimate the amount of vegetation is by judging "canopy cover." This is the percentage of a given surface area which is shaded by vegetation when viewed from directly above. Generally, the greater the percentage of canopy cover, the better the protection provided (compare Figures 2-20 and 2-21). Assuming desirable species are present, the greater the canopy cover, the higher the productivity of the vegetation.

Not all plant species are equally useful from a management perspective. The presence of noxious weeds or other aggressive invader species may be symptomatic of a riparian area with problems. They reduce habitat diversity and quality as well as reducing production of desirable species. Although undesirable species such as dandelion, cheatgrass, Kentucky bluegrass, small clovers, plantain, and pussy-toes are not considered as bad as noxious weeds, their presence may indicate a trend away from the site's potential. Often they indicate a high level of disturbance. Moreover, they are usually low-producing species that provide only limited soil- and bank-holding capability.