"Many die crossing the Mara River," Estes told The Dodo. Estes, who is also a research associate at the Smithsonian Institution, began studying wildebeest in 1962. "They die all over the range, but the Mara is where thousands of wildebeest (gnus) converge. It is also where thousands of tourists go to view and photograph the crossings."
"In fact," Estes notes, "sometimes so many tour vehicles line up along the banks, they block the wildebeest who have crossed from the other side."
There are some 1.25 million wildebeest in the region, according to Estes. Although the mass drowning of animals makes for astonishing images, the annual birth of up to a quarter-million calves, he explains, offsets all the losses and keeps the wildebeest numbers robust.