What is a district judge's duty in connection with a report and recommendation?

Federal Rule of Civil Procedure 72(b) and 28 U.S.C. 636(b)(1) set forth a district judge's duties in connection with a magistrate judge's report and recommendation. The district judge must "make a de novo determination of those portions of the report . . . to which objection is made," and "may accept, reject, or modify, in whole or in part, the findings or recommendations made by the magistrate judge." 28 U.S.C. 636(b)(1)(C); see also United States v. Remsing, 874 F.2d 614, 617-18 (9th Cir. 1989).