The Central Valley is a geological trough, 450 miles long and 40 to 60 miles wide, hemmed in by the Sierra Nevada to the east and the Coast Ranges to the west. Its floor is flat or gently rolling, sloping toward the rivers that once braided across it without constraint. The Sacramento River drains the northern half, flowing south; the San Joaquin River drains the southern half, flowing north. They meet in the vast inland delta east of San Francisco Bay. Before dams and levees straightened these rivers, annual floods deposited rich alluvial soil across the valley floor and recharged the wetlands, vernal pools, and seasonal lakes that defined the landscape. The valley's Mediterranean climate, with cool wet winters and hot dry summers, created growing conditions unlike anywhere else on the continent. Native perennial bunchgrasses like purple needlegrass thrived during the rainy season, going dormant in summer. The entire grassland ecosystem was tuned to this rhythm of flood and drought.

The grasslands that now blanket the valley floor are mostly imposters. European annual grasses, introduced during the Spanish colonial period, have displaced the native perennials across most of the ecoregion. Species like wild oats, soft chess, and ripgut brome germinate with the first autumn rains, grow rapidly through winter, set seed in spring, and die by summer, leaving behind a brown stubble that fuels wildfires. This cycle favors the annuals over the slower-growing native bunchgrasses, which need multiple seasons to establish. The transformation was swift and thorough. Within a few decades of European contact, introduced grasses had colonized nearly the entire valley. Overgrazing by cattle accelerated the shift, as livestock preferentially ate the native grasses and left the less palatable invaders standing. Today, native grasslands survive only in fragments, mostly on rocky outcrops, alkaline flats, or protected refuges where conditions discourage the introduced species.

The most fragile and distinctive habitats in the ecoregion are its vernal pools. These seasonal wetlands form in shallow depressions where underlying clay or hardpan prevents water from draining, creating pools that fill with winter rain and dry completely by late spring. Each pool supports a community of plants and animals found almost nowhere else, including fairy shrimp species so geographically restricted that individual pool complexes harbor unique genetic lineages. When the pools begin to dry in March and April, wildflowers germinate in concentric rings around the receding waterline. The effect can be spectacular: carpets of goldfields, lupines, California poppies, and purple owl's clover spreading across the grasslands in blooms so vivid that they are visible from aircraft. John Muir, crossing the valley in the 1860s, described it as the floweriest piece of world he had ever walked through. The blooms still appear each spring, especially in the Carrizo Plain and the Antelope Valley south of the main valley floor, but their extent has diminished as vernal pools are plowed under or paved over.

The list of what has vanished from the Central Valley reads like an inventory of a different continent. Tulare Lake, Buena Vista Lake, and Kern Lake are dry. The tule elk that once grazed in herds of hundreds of thousands were reduced to a single pair by 1870. Grizzly bears, which gave California its state flag emblem, were hunted to extinction in the valley by the 1920s. Pronghorn antelope, once common on the grasslands, disappeared from the region entirely. More than 95 percent of the valley's original wetlands have been drained, and the native grasslands that survived plowing have been overrun by introduced species. Yet the valley has not been emptied. The managed refuges along the San Joaquin River still host millions of wintering waterfowl. Tule elk have been reintroduced to scattered sites. Vernal pool preserves protect the rarest wildflower communities. The ecoregion endures, diminished but not erased, its remaining fragments a reminder that the richest grassland in the American West was here long before the orchards and the cotton fields.