Rumex, Wild rhubarb

Here's a closeup of the flower heads.

Rumex hymenosepalus Polygonaceae (Buckwheat family) Common names: Wild Rhubarb, Desert Rhubarb, Canaigre, Dock Location: Glen Canyon near Bullfrog. Open desert, sandy, with slickrock. April 12, 2008. Elevation: approx 4000 feet NAD83 UTM: 12S 530250 mE 4164000 mN Dominance: Uncommon in the area. Comments:   Source: Spellenberg, Richard. (1979, 1994) National Audubon Society Field Guide to North American Wildflowers, Western Region. Alfred A. Knopf, New York. ISBN 0-394-50431-3. Description page 679.