 Work started at Carnarvon , and 190 miles ( 310 km ) had been completed by 1969 . Locally available material was used to great extent during construction , with techniques adjusted based on what was available ; however , some resources were transported across vast distances . Water , scarce in the dry climate of the North West , was conveyed up to 15 miles ( 24 km ) , aggregate up to 100 miles ( 160 km ) , and bitumen up to 850 miles ( 1 @,@ 370 km ) . The project required thirty bridges , over riverbeds that could be dry all year , but might receive as much as 1 @,@ 300 millimetres ( 51 in ) of rainfall in three months , as had been the case at Onslow in 1961 . The bridges were designed to withstand twenty @-@ year floods , but more severe possibilities were anticipated – the approaches were built lower than the bridge decks , so that excess water would flow around the bridges , rather than over them . 
