Sundown Lake is a private, man-made lake in Appanoose County, Iowa, United States, 8 miles (13 km) east of the much larger Rathbun Lake, 2 miles (3.2 km) northwest of Unionville and 7 miles (11 km) southeast of Moravia, Iowa.

It was created in 1976, by building a 51 foot tall, 1530 foot long embankment dam on the South Soap Creek, a tributary of the Soap Creek, which is itself a tributary of the Des Moines river.

The lake originally covered about 470 acres, but silting has reduced its size.

[4][5] The Appanoose County Board of Supervisors fought unsuccessfully to prevent the establishment of the RIZ, claiming that the diversion of tax revenues would adversely impact schools and local governments.

[2][6] After losing a court battle,[7] the Board of Supervisors approved the creation of the RIZ on June 2, 2014.