That's according to the Grant County Public Utility District on Tuesday.
The crack formed across a spillway of the huge dam, and may have been exacerbated by a weak construction joint.
The preliminary findings will be submitted to the utility's Board of Consultants for final review.
The utility says a team of engineers discovered the mathematical error while examining the original design calculations of the Wanapum Dam spillway. When they recalculated the design formulas, they found that additional concrete and/or reinforced steel should have been used in the construction.