7423 - Laserfiche WebLink

Home Browse Search

48 I I I I I I I I I I I I I I I I I I I In the impact matrices (Figures VI-5 and VI-7) which follow, a "plus" indicates that the critical resource is positively affected by adding flood (or fluctuating) releases to the baseline sequence of years. A "minus" indicates that the addition of floods or fluctuations has a significant negative impact. A "zero" indicates no significant impact, and a question mark indicates that the current data are insufficient to judge impacts. Flood Releases Have Negative Impacts On Terrestrial Resources And Recreation The flowchart in Figure VI-2 shows the pathways by which flood releases adversely impact the critical re- sources. As the magnitude, frequency, and duration of floods increase, the impact of floods on resources also increases. These impacts are displayed for each critical resource in the matrix in Figure VI-5. Terrestrial resources. Floods are generally deleterious to downstream resources, but their greatest negative impact occurs to terrestrial resources and recreation. Beach sand is redistributed and may be lost from the system whenever flows inundate areas normally exposed. Although some beaches, especially in wide reaches of the river, may build up as a result of the redistribution of sand, these new deposits are rapidly eroded after flood recession (Appendix A, Section II). Sand deposits used as camping beaches are typically more protected from erosion than other sand deposits. However, loss of sand from less protected deposits will result in gradual loss of camping beaches because these less protected deposits supply sand to replenish camping beach deposits (Appendix A, Section II). As flow increases above 40,000-50,000 cfs, more and more of the beaches protected by debris fans are subjected to erosive downstream flow. At 70,000-90,000 cfs, most sand deposits are subject to direct erosion by downstream flow (Appendix A, section II). The impact of floods on beaches is greatest upstream of the Little Colorado River, which is the major source of new sand to Grand Canyon. The loss of camping beaches and sand substrate is potentially irreversible because sediment lost to the system during flooding is not quickly replaced by tributary flows.