A combination of design and construction flaws, continuing to operate the dam when the primary system for gauging the water level was known to be inaccurate (gauge pipes had become detached), moving the "failsafe" secondary gauging system above the actual height of the dam to avoid false positives, and operating the dam in an unsafe manner by routinely overfilling the reservoir caused the upper reservoir dam to overtop.http://www.bpa.gov/power/PG/NW-HydroOperators-Forum/Materials/CaseStudyT-Sauk_Ehasz-Paul.pdf Missouri University of Science & Technology: The 2005 Upper Taum Sauk Dam Failure:A Case History There was no overflow spillway in the original reservoir.
