Human activities have replaced or altered over 95 percent of the historic wetlands as well as the California oak woodland habitat, which originally occurred along stream and river corridors in the foothills, and the tule grass that once thrived in huge stands on the edges of marshes and lakes. Some of the richest remaining marsh habitats are in the Sacramento-San Joaquin Delta, which despite significant agricultural and infrastructure development, has retained many of its original swamps and backwaters. Before the 19th century, the delta was a region of numerous islands of nutrient-rich peat, alluvial deposits, winding channels and waterways. Since then, most of the Delta islands have been cultivated, and consistent use of the land has resulted in subsidence, in some cases up to . Water diversions from the rivers feeding it have increased salinity, which has in turn caused declines in fish populations that once thrived in the region.