This clip, from Disney’s 1958 feature White Wilderness, is incorrect. It’s been largely credited with the spread of the lemmings-jump-to-their-deaths myth. Lemmings do not commit mass suicide, no matter what Disney or any video game suggests. In fact, that particular sequence was largely faked by the film crew using a handful of individual lemmings who were herded off the cliff into a river.
What does happen is that, during population booms, individual lemmings will sometimes accrue into mass migrations which will seek out new areas. And some do try to cross rivers or streams and end up drowning. That’s about it.
Although, fun fact, lemmings are not the only rodents who do this. In the 1800’s it was fairly common for people living in the United States to see similar runs from the local northern grey squirrel. It was quite a sight. John James Audubon saw one or two of these migrations as they crossed over rivers and streams and wrote on it, saying:
Onward they come, devouring on their way every thing that is suited to their taste, laying waste the corn and wheat-fields of the farmer; and as their numbers are thinned by the gun, the dog, and the club, others fall in and fill up the ranks, until they occasion infinite mischief, and call forth more than empty threats of vengeance. It is often inquired, how these little creatures, that on common occasions have such an instinctive dread of water, are enabled to cross broad and rapid rivers, like the Ohio and Hudson for instance. It has been asserted by authors, and is believed by many, that they carry to the shore a suitable piece of bark, and seizing the opportunity of a favourable breeze, seat themselves on this substitute for a boat, hoist their broad tails as a sail, and float safely to the opposite shore. This together with many other traits of intelligence ascribed to this species, we suspect to be apocryphal… those which we observed crossing the river were swimming deep and awkwardly, their bodies and tails wholly submerged; several that had been drowned were carried downwards by the stream; and those which were so fortunate as to reach the opposite bank were so wet and fatigued, that the boys stationed there with clubs found no difficulty in securing them alive or in killing them.
At times they were strewed, as it were, over the surface of the water, and some of them being fatigued, sought a few moments’ rest on our long “steering oar,” which hung into the water in a slanting direction over the stern of our boat. The boys, along the shores and in boats, were killing the squirrels with clubs in great numbers, although most of them got safe across. After they had reached the shore we saw some of them trimming their fur on the fences or on logs of drift-wood.
The migrations seemed to have stopped with the trimming of North America’s forests.