Construction has started on a $7.3 million dam rehabilitation project that will close a busy fishing destination in American Fork Canyon for the rest of the year. Federal authorities have drained Tibble Fork Reservoir and are replacing its half-century-old earthen dam with a structure that is 15 feet higher and enlarging the reservoirâs surface area from 9.8 to 21.6 acres, according to the Natural Resources Conservation Service (NRCS).