What I have always referred to as Whitewater Canyon is actually the Whitewater Preserve, located in the San Bernardino Mountains within the Sand to Snow National Monument, and centered on the Whitewater River and its steep canyon walls. Wikipedia adds this: "The Preserve sits where the Mojave Desert and Sonoran Desert meet. The Whitewater River flows year-round from San Gorgonio Mountain through the canyon, although after major storms debris sometimes buries the surface channel causing flows to run underground until the riverbed reforms and snowmelt or groundwater recharge and restore visible flow. The canyon is also subject to periodic flooding after heavy mountain rains, which can alter channels and affect access. The Preserve is surrounded by the Bureau of Land Management’s San Gorgonio Wilderness and forms a wildlife corridor to the San Jacinto Mountains. Whitewater River is part of the National Wild and Scenic Rivers System, designated in 2019 under the John D. Dingell Jr. Conservation, Management, and Recreation Act." The picture above shows the meandering stream channel with the San Bernardino Mountains in the distance. There are trails up canyon on both sides of the stream channel. In the 1930s a trout farm operated in the canyon and in 2006 Friends of the Desert Mountains purchased it with state bond funds through the Coachella Valley Mountains Conservancy and transferred it to The Wildlands Conservancy for permanent management. The main trail in the Preserve is the 4-mile Canyon Loop Trail, but there is also access into the San Bernardino Mts via the PCT. The website of the Wildlands Conservancy provides this: "The canyon has a robust population of bighorn sheep, deer and bear, and is an important wildlife corridor between the San Bernardino and San Jacinto Mountains. The Wildlands Conservancy purchased an additional 3,200 acres in the Whitewater corridor that were donated to the Bureau of Land Management. These donated lands include sand dunes that are home to the endangered fringe-toed lizard at Windy Point, which lies at the confluence of the Whitewater and San Gorgonio Rivers." To reach the Whitewater Preserve, take exit 114 from the I-10 freeway to Tipton Road, then turn left on Whitewater Canyon Road. At the present time Whitewater Preserve is open after several closures resulting from storm damage from Tropical Storm Hilary in August 2023 which caused extensive damage to roads and trails, but it's recommended to check the Preserve's official website for the most up-to-date status before visiting. Mission Creek Preserve is owned by the Wildlands Conservancy and is immediately adjacent to Whitewater Canyon and consists of 4,670 acres at the center area of which is an unusual for the area lush wetland. To get there, go past Whiewater and continue on 62 to Mission Creek Road. This gallery is based on 19 visits between 2006 and 2010, and includes photos taken in the canyon, on the hillsides above the canyon proper, and in Mission Creek Preserve. I should indicate that I can't guarantee the accuracy of all of the identifications. Tom Chester's plant voucher checklist for Whitewater Canyon is here and for Mission Creek Preserve here. As usual an asterisk next to the common name indicates a non-native taxon.

Brittlebush
Encelia farinosa
Asteraceae
[Named for Christoph Entzelt, 1517-1583]

Fremont's pincushion
Chaenactis fremontii
Asteraceae
[Named for John Charles Frémont, 1813-1890]

Heartleaf sun cup
Chylismia cardiophylla ssp. cardiophylla
Onagraceae

Glandular theadplant
Nemacladus glanduliferus
Campanulaceae

Retrorsre desert four o'clock
Mirabilis laevis var. retrorsa
Nyctaginaceae

Beavertail cactus
Opuntia basilaris var. basilaris
Cactaceae

California croton
Croton californicus
Euphorbiaceae

Bird's-foot fern
Pellaea mucronata var. mucronata
Pteridaceae

White rhatany
Krameria bicolor
Krameriaceae
[Named for Johann Georg Heinrich Kramer, 1684-1744]

White nightshade
Solanum douglasii
Solanaceae
[Named for David Douglas; 1798-1834]

Parish's goldeneye
Bahiopsis parishii
Asteraceae

Small-seeded spurge
Euphorbia polycarpa
Euphorbiaceae

Silver lotus
Acmispon argophyllus var. argophyllus
Fabaceae