Figure 3. Photographs showing aeolian sand dunes in Grand Canyon. Dunes in (A) and (B) have modern sand supply, being situated directly downwind of fluvial sandbars that are replenished by occasional controlled flooding. At the right side of (A) a weather station is visible, one of several used to measure wind conditions in the canyon. Dunes in (C) and (D) do not receive sand supply in the post-dam era, because no floods have occurred that were large enough to supply sand to these areas or to places upwind of them. Without recent sand supply, dunes in (C) and (D) have been colonized by dark-colored biologic soil crust.
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