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The reasons for the collapse of the St. Francis Dam were only realized in retrospect, but the lessons learned have advised construction of large dams to the present day.

The St. Francis Dam was built in an area prone to earthquake faults and unstable ground. Although science at the time could not have detected it, there was an underlying, ancient landslide, composed of layers of mica schist at a 45 degree angle, underneath the east end of the dam. This landslide was stable and unmoving until the weight of the dam, and lubrication of the reservoir water seepage, caused it to lift and slide.

Cracks in the dam, inspected by Mulholland up till the day before the collapse, were deemed to be unremarkable temperature and pressure cracks. However, their 45 degree angle indicated a shifting dam base.

The dual additions to the dam, increasing height by 11%, were not compensated for by an increase in dam width, contributing to its instability.

Officials decided, without laying blame on Mulholland, that never again would decisions around the design and construction of a large dam project be left to one person, regardless of their capabilities or experience.