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100 80 60 ] ~ 40 20 o 200 180 ~ Bedrock mBl Talus _ Pre-Glen Canyon Dam fluvial deposits _ Eolian deposits m Tributory fan fluvial deposits _ Fluvial deposits actively reworked since Glen Canyon Dam Zone B 160 140 120 100 Feet Figure 4. The cross section shows an ideal- ized river deposit under post-dam condi- tions. The characteristics of the vegetation, Zone A 12~.000 dS'!] Pre-dam flood terraces 80,000 ds? 80 Highest contemporary discharge (30,000 ds) Daily high water (20,000 ds) Daily low water (4,600 ds) o 60 40 20 sediments, and human use of the 3 zones are listed in Table 2. Effects of Glen Canyon Dam points where the river is constricted by the debris fans of tributary washes (Fig. 6). Immediately above and below the falls, a lower-velocity .reverse eddy caused deposition of fluvial terraces that mantled the debris fans. of flood peaks (see Table 1). Since demands for hydroelectric power determine the schedule of dis- charges, the discharge varies by a factor of about 5 over a 24-hour cycle, resulting in a vertical daily variation of the river by as much as The regulation of flow by Glen Canyon Dam has resulted in a slight increase in median discharge and a great decrease in the number Table 2. Characteristics of beach zones in Figure 4 Substra te Dominant grain size of substrate V egeta tion density Common species on fine-grained fluvial deposits Human use Undesirable human impact Zone A Post-dam fluvial sediments Fine sand Sparse to dense Pluchea sericea (arrow weed) (S) Tamarix pentandra (tamarisk) (SE) Salix exigua (coyote willow) (S) Cynodon da.ctyion (Bermuda grass) (E) Mooring, bathing; high portion occasionally used for camping and thus for cooking, etc. Ephemeral but probably contributes to accelerated erosion (E) = exotic invader (S) = shrub (T) = tree (Species identification by Roy Johnson) Zone B Zone C Pre-dam fluvial sediments reworked by eolian processes High pre-dam flood terraces and eolian deposits Fluvial: fine sand and silt Eolian: fine sand Fluvial: fine sand and silt Eolian: fine sand Sparse to dense Dense Pluchea sericea (arrow weed) (S) Bromus rubens (red brome) (E) Alhigi camelorum (camel thorn) (SE) Salsola kali (Russian thistle) (E) An unidentified composite Fallugia paradoxia (Apache plume) (S) Acacia greggii (cat-claw acacia) (ST) Prosopis juliflora (honey mesquite) (ST) Baccharis sarothroides (desert broom) (S) Camping, cooking, disposal of human wastes Little use because of dense vegetation, distance from river, steep slopes Accumulation of scraps and chemically treated human waste; damage to vegetative cover and soil leading to wind erosion; direct erosion by man along paths and on steep slopes Little impact I 396 American Scientist, Volume 62