Before the arrival of European settlers in the 19th century, much of the Sacramento valley was taken up by seasonal
wetlands and
grasslands. By the beginning of the 20th century, much of this had been replaced by farmland, particularly for the growing of
rice, and the rivers no longer create new wetlands because their flow is controlled by
levees and irrigation schemes. Less than 10% of the original wetland area remains. Migrating birds have continued to use the area, and resting in the rice fields, consumed considerable quantities of the crop. In 1937, the
United States Fish and Wildlife Service, with the aid of the
Civilian Conservation Corps, began the process of creating a refuge within dry, alkaline lands between the towns of
Willows and
Maxwell. This was the original Sacramento NWR. From the 1940s onward, additional refuges were created, so that the Sacramento NWR Complex now includes the following refuges, located between north of the city of
Sacramento: at Sacramento National Wildlife Refuge • Sacramento National Wildlife Refuge (southeastern
Glenn and northeastern
Colusa counties, south of
Willows) •
Delevan National Wildlife Refuge (east of Maxwell) •
Colusa National Wildlife Refuge (between the towns of Colusa and
Williams) •
Sutter National Wildlife Refuge (southwest of
Yuba City) •
Sacramento River National Wildlife Refuge (several non-contiguous sites along the Sacramento River between
Red Bluff and
Colusa) •
Butte Sink Wildlife Management Area (northeast of the town of
Colusa) • North Central Valley Wildlife Management Area • Willow Creek-Lurline Wildlife Management Area The total area of the refuges is about s). ==Ecology==