EMAP design and River Reach File 3 (RF3) as a sample frame in the Central Valley, California

The Central Valley, California, R-EMAP project assessed the effects of high ly modified, agriculturally dominated landuse on the aquatic resources of t he lower portion of the Central Valley watersheds. The focus of this paper is to assess the utility of the EMAP design and the River Reach File versio n 3 (RF3) 1:100,000 scale Digital Line Graph (DLG) as a sampling frame. The study area is 34.099 mi(2) (88,316 km(2)) and comprises the lower reaches of the Sacramento River and San Joaquin River watersheds to the 1000 ft. (3 05 m) elevation, Sampling sites are selected using a tessellation stratifie d design to represent the two main populations of interest: natural streams and man-made waterways, Sites are selected to represent 13,226 miles of st reams and sloughs, and 14,648 miles of irrigation canals, ditches and drain s. To achieve an approximately equal sample size across stream orders and b asins. the sample design was weighted by Strahler order categories to ensur e sampling occurred in the higher order streams. Based on office and field reconnaissance, the study provided information on the quality of RF3 as a s ampling frame. Site selection using RF3 had a success rate of approximately 44%. The RF3 database has an error rate of approximately 7%. When human in fluence factors were included, the error rate increased to 16%. There was a n 11% error rate when selecting sites for natural streams, and approximatel y a 14% error rate for man-made waterways. The reconnaissance information i ndicated that presence or absence of irrigation ditches and return drains d epends on changing agricultural uses. Some of the error in the RF3 for natu ral streams and man-made waterways can be attributed to rapid urban expansi on, especially in the San Joaquin basin.