Scale effects on interpretation of riparian vegetation monitoring data from the Colorado River in Grand Canyon

Two tributaries, the Paria and Little Colorado Rivers, are significant sources of sediment, and nutrient pulses for the Colorado River in Grand Canyon. Analysis of riparian vegetation for the Colorado River in the 1990s identified that marsh community composition reflected geomorphic reach (canyon width) and system-wide effects, including daily inundation patterns associate with Glen Canyon Dam operations. A monitoring program evaluating change in total cover, total species richness and diversity for the Colorado River in Grand Canyon from 2001 through 2004 across five surface elevations (424, 708, 991, 1274, 1699 m³/s) identified yearly operations, local precipitation and geomorphic reach as significant variables influencing these three vegetation measures. The results indicated that the direction of response differed among surface elevations. These research and monitoring efforts did not consider tributary effects.  Because the timing and duration of a tributary input affects available substrate for colonization, nutrient dynamics, seed bank inputs and subsequent species diversity, it may be an unaccounted-for variable affecting vegetation change, and adds another geographic scale of effects on data interpretation. Preliminary results from reanalysis of the monitoring data for an area bracketed between the Paria and Little Colorado Rivers (97 km in length) indicate richness and diversity did not differ among surface elevations across years, and that the effect of geomorphic reach varied across years (F_{richness(4,322)} =12.46, p<0.0001; F_{diversity(4, 322)}=8.98, p<0.0001). These results and others related to the effect of scale on monitoring approaches and interpretation in a regulated river system are discussed.