Although the fur traders interested in beaver and river otter pelts did not spend many years in the Delta, their effect was extreme. The first arrived in 1826, and by 1833 not only were the furbearer populations decimated, but the trappers had introduced malaria to the valley. It is estimated that in 1833 alone, 20,000 Native Americans died of the disease, and between 1833 and 1846 as much as 75% of the native population succumbed to subsequent waves of infection. This left the survivors completely unable to resist the waves of settlers arriving with the Gold Rush and forced the collapse of the native cultures.