USE OF REPEAT PHOTOGRAPHY AND OTHER TECHNIQUES TO UNDERSTAND RIVER MORPHOLOGY AND RIPARIAN VEGETATION CHANGES ALONG A CHANNELIZED REACH OF THE NORTH FORK OF THE VIRGIN RIVER IN ZION NATIONAL PARK

In the 1930’s the National Park Service straightened, entrenched and armored the channel of the North Fork of the Virgin River in Zion National Park. The need for this action was apparently clear at the time, but is no longer obvious, and the park has proposed removing the levees to restore the river to a more natural function. Investigations have been undertaken to address questions such as, why the river was so aggressively channelized, why the valley floor has an extensive canopy of senescent cottonwood trees, and what is meant by restoration when river morphology has changed dramatically since the time of channelization. Repeat photography of images made in the early 20th century has shown that the river morphology has changed dramatically, from a very wide, braided and active channel, to a much narrower, single channel. While such shifts are expected where the river is confined by levees, this shift in morphology has occurred in unchannelized reaches and on the similar East Fork of the Virgin River which has never been channelized. Such a widespread shift is apparently the result of changes in patterns of flooding, with a possible influence from changes in watershed condition.

Use of repeat aerial photography from 1952 to 1994 shows that where the river breaks through the armored levees, lateral migration of the river has occurred at a rate of 4.7 feet/year to the west and 3.4 feet/year to the east, with the rate of sediment removal found to be 230 cubic-yards/year to the west, and 850 cubic-yards/year to the east.

This understanding greatly influences National Park Service expectations for a restored river. Restoration of a more natural meander pattern and functioning floodplain will not immediately threaten park facilities. In addition, even though an unconfined channel will enhance cottonwood recruitment, it will not replace the senescent cottonwood canopy that covers the valley floor.