From Lakota America: A New History of Indigenous Power, by Pekka Hämäläinen (The Lamar Series in Western History; Yale U. Press, 2019), Kindle pp. 167-169:
Oglalas and Sicangus were the first to turn west, ascending the White and Bad [rivers] which offered reliable water and wood, serving as superb passageways. As they inched upriver, they clashed with Kiowas and Crows, pushing the former to the south and the latter to the west. Sicangus joined forces with Cheyennes against the Kiowas, while Oglalas spearheaded campaigns against Crows under the leadership of Bull Bear whose boldness and resolve seemed to have given their operations particular sharpness. By the late 1820s Crows had retreated from the Black Hills into the Powder River country a hundred miles to the west, and Oglalas and Sicangus established themselves on Pahá Sápa’s eastern side. By that time Sans Arcs, Minneconjous, Two Kettles, Sihasapas, and Hunkpapas had also begun to shift west, establishing the Cheyenne and Grand [rivers] as entryways.
Lakotas had turned west, decisively, but they remained people of river valleys first and people of the plains second. They ventured into the western grasslands and Pahá Sápa from multiple points along timbered river valleys and moved back regularly along those same valleys to the Mníšoše [Missouri R.], which remained vital to their cosmology and economy. Essentially protrusions of the Missourian riparian woodlands, the White, Bad, Cheyenne, and other tributaries provided Lakotas with safe and familiar pathways into the West. When they began pulling away from the Missouri, they were not so much casting themselves loose on the open plains but extending their old riverland world into the West.
This tripartite pattern—a trunk line in the east, an elevated, magnet-like anchor in the west, and a row of arterials in between—formed the core of the Lakota world from the 1830s onward. It was in those three places where Lakotas spent most of their time—cooking, eating, sleeping, socializing, smoking, praying, raising children, tending horses, preparing hides, making clothes, tools, and weapons—and where most of them entered and left this world. This was the homeland Lakotas would defend against invaders and, when needed, expand.
The western tributaries were keys to land and wealth and power, but they were also conduits into a perilous new world, for they carried Lakotas toward greater aridity. The western Great Plains lie in a long rain shadow cast by the Rocky Mountains. Pacific winds pump moist maritime air eastward, but the air sheds much of its moisture while climbing up the Rockies. The shadow effect is strongest in the west, the 98th meridian marking a default line where evaporation exceeds precipitation and the soil starts to go dry. The grass cover reflects this, becoming shorter toward the west. Around the 105th meridian densely tufted blue grama and buffalo grasses become dominant and the plant canopy shrinks down to a few inches. The bison had adapted over the millennia to these semi-arid conditions, thriving on the stunted short-grasses that retained protein in their dry stalks, making them ideal winter forage. Tens of millions of them lived in the plains, their huge bodies a hunter’s delight.
That was when the Black Hills began to loom large in a material sense. When severe droughts struck the grasslands, the bison tended to seek relief in the high-altitude microclimate around the Black Hills where summers were cooler, rainfall higher, and pasture more lavish. Lakotas did the same. This introduced a new dimension to Pahá Sápa’s allure: it became a sanctuary and a meat pack. Lakotas gathered there to sit out droughts and subsist on buffaloes that seemed to give themselves up—just like the White Buffalo Calf Woman had promised. The control of Pahá Sápa became a spiritual and material imperative without which nothing in the world was secure—not its unearthly bounties, not the hunt, not the survival of Lakotas as a people. Hand paintings on Pahá Sápa’s rocky planes—red spots marking slain enemies—bespeak of the violent struggle that turned the mountain range into an exclusive domain of Lakotas and their Cheyenne and Arapaho allies. By the late 1820s Lakotas were wintering in Pahá Sápa. The Black Hills belonged to them.