Editing Whitewater (section)

=== Undercut rocks ===
Undercut rocks have been worn down underneath the surface by the river, or are loose boulders which cantilever out beyond their resting spots on the riverbed.  They can be extremely dangerous features of a rapid because a person can get trapped underneath them under water.  This is especially true of rocks that are undercut on the upstream side.  Here, a boater may become pinned against the rock under water.  Many whitewater deaths have occurred in this fashion.  Undercuts sometimes have pillows, but other times the water just flows smoothly under them, which can indicate that the rock is undercut.  Undercuts are most common in rivers where the riverbed cuts through sedimentary rocks such as [[limestone]] rather than [[igneous rock]] such as [[granite]].  In a steep canyon, the side walls of the canyon can also be undercut.

A particularly notorious undercut rock is Dimple Rock, in Dimple Rapid on the Lower [[Youghiogheny River]], a very popular rafting and kayaking river in Pennsylvania.  Of about nine people who have died at or near Dimple Rock, including three in 2000, several of the deaths were the result of people becoming entrapped after they were swept under the rock.<ref>{{cite web |url= http://www.post-gazette.com/pg/06100/680887-192.stm |title= Editorial: Rock on / Dimple Rock is left alone to be dangerous
|access-date= 2008-02-29 |date= 2006-04-10 |publisher= Pittsburgh Post-Gazette}}</ref>
<ref>{{cite web |url= http://www.americanwhitewater.org/content/River/detail/id/1687/#tab-news |title= American Whitewate, Lower Yough accident reports|access-date=30 December 2010|publisher=Americanwhitewater.org}}</ref>