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the dimensions and areas of the digitized habitat polygons. The interpreted rasters are stored on rewriteable optical disks. The 19-inch color monitor is viewed during the interpretation process to assist the video analyst because it has higher resolution than the computer monitor. A section of tape is played and reversed as needed during the interpretation to take advantage of changing sun reflection and perspective. One advantage videography has over aerial photography for river habitat quantification is its ability to observe motion. The ability to see the velocity of the water is a great aid, for instance, in distinguishing a side channel that marginally qualifies for that designation from the main channel. Change in perspective is another advantage videography has. This change in perspective is helpful in the case of overhanging vegetation where the changing position may allow identification of otherwise hidden features. Also, it causes the sun glint to travel along the water, making shallow water easier to distinguish from wet sand. Interpretation River habitat interpretation requires development of basic criteria for categorizing each of the different habitat types. Because a wide variety of subtle variations occur within habitat types, especially side channels, the initial definition has to be refined as more and more variations are encountered. The following habitat categories were interpreted from the 1991 videography: . Backwaters . Side channels . Isolated pools . Main channel The main criterion used for defining a backwater was that the velocity of the water was zero. A backwater could have only one connection to a main channel or side channel so no flow through occurred. A backwater could occur along the bank of a main channel or side channel. They also could occur in islands or sandbars within the main channel or side channels. Figure 2 shows examples of backwater fonn. Side channels observed on the 1991 videography of the San Juan River included small ephemeral side channels 40 square meters in area to large, nearly pennanent features with areas of 90,000 square meters. The primary criteria used to identify side channels were: . Width ofless than 50 percent of the main channel . Reduced flow velocity and volume · Course deviated from the main channel and then rejoined · The angle at which the channel deviates from the main channel was not streamlined (greater than 45 degrees) with respect to the main channel flow. These criteria worked well for identifying the larger classic side channels (secondary channels). Smaller, less well-defined, and ephemeral side channels were also identified by widths 4