From the HSUS - Horses Plunge to Death in Suicide Race

Every summer, the town of Omak, Washington, bears witness to a cruel and deadly horse racing ritual: the Omak Suicide Race. In 2004, three horses died after hurtling down a mountainside, literally at breakneck speeds. Each year, the protests over the cruelty grow, but still the event persists and animals die.
Since 1983, there have been 21 documented deaths. Although the Omak Suicide Race originated as an offshoot of the Omak Stampede, an annual rodeo, many do not recognize the race as a normal or customary rodeo event. Last year, HSUS staff, accompanied by veterinary professionals, witnessed this deadly “race.”
The opening stretch of the course is a plunge down a 60-degree slope that is 210 feet in length and covered with loose, wet sand. Riders race the horses towards this sharp drop at a full gallop, an almost sure invitation for broken bones and other injuries. Most of the human and horse injuries and deaths occur on this initial stretch down "Suicide Hill," constituting the "thrilling" excitement of the race.
Horses who survive the initial, vertical plunge down Suicide Hill are immediately forced to jump into the unpredictable waters of the Okanogan River. Some horses actually tumble down the mountain, land in the water sideways, and have to right themselves to avoid drowning. The exhausted and terrified horses are then forced to swim or run, depending on the depth of the river, a span of approximately 50 yards, where more than one horse has drowned over the years.
The race does not end on the far bank. As the animals struggle out of the river, riders whip and kick the horses into a gallop up a steep grade to the finish line. By the end of the final uphill sprint, the surviving animals stand panting and exhausted, their coats wet with sweat and the river water. Not as easy to see is the mental stress the race has on the horses—by nature high-strung, sensitive animals.
The horses and riders who successfully complete the first race will go on to race again the second night, and then a third and a fourth race, as long as they successfully complete—and survive—the previous races. Last year, by day three, nearly half of the horses were unable to continue and were eliminated from the race.