Laserfiche WebLink
catostomids tend to dominate the fish fauna. Primary production is <br />highest in this zone because of higher nutrient levels and reduced <br />turbulence. <br />The lower zone has two distinct subunits: the canyon areas of <br />steep gradient, and meandering river sections in flat terrain with <br />canyons of low gradient. Water in this zone is generally warm and very <br />turbid. Substrate types in steep gradient canyons alternate between <br />sand and gravel rubble; in the level, meandering flats and low-gradient <br />canyon areas, the substrate is almost totally sand. Primary production <br />is virtually absent due to low water clarity and substrate scouring, and <br />benthic invertebrate abundance?is directly related to the availability <br />of gravel-rubble substrate. The major energy source for aquatic commu- <br />nities in this zone is allochthonous materials. Fish species composition <br />is generally similar in the two subunits; catostomids and cyprinids <br />dominate, but relative abundance of the various species differs substan- <br />tially. <br />6