 Tunneling began at the lower portals of the Nevada tunnels in May 1931 . Shortly afterward , work began on two similar tunnels in the Arizona canyon wall . In March 1932 , work began on lining the tunnels with concrete . First the base , or invert , was poured . Gantry cranes , running on rails through the entire length of each tunnel were used to place the concrete . The sidewalls were poured next . Movable sections of steel forms were used for the sidewalls . Finally , using pneumatic guns , the overheads were filled in . The concrete lining is 3 feet ( 1 m ) thick , reducing the finished tunnel diameter to 50 ft ( 15 m ) . The river was diverted into the two Arizona tunnels on November 13 , 1932 ; the Nevada tunnels were kept in reserve for high water . This was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into the river until its natural course was blocked . 
 Tunneling began at the lower portals of the Nevada tunnels in May 1931 . Shortly afterward , work began on two similar tunnels in the Arizona canyon wall . In March 1932 , work began on lining the tunnels with concrete . First the base , or invert , was poured . Gantry cranes , running on rails through the entire length of each tunnel <a href='uninflect' title='1'><mark>were</mark></a> used to place the concrete . The sidewalls <a href='uninflect' title='2'><mark>were</mark></a> poured next . Movable sections of steel forms <a href='uninflect' title='3'><mark>were</mark></a> used for the sidewalls . Finally , using pneumatic guns , the overheads <a href='uninflect' title='4'><mark>were</mark></a> filled in . The concrete lining <a href='drop_aux' title='5'><mark>is</mark></a>3 feet ( 1 m ) thick , reducing the finished tunnel diameter to 50 ft ( 15 m ) . The river was diverted into the two Arizona tunnels on November 13 , 1932 ; the Nevada tunnels <a href='uninflect' title='6'><mark>were</mark></a> kept in reserve for high water . This was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into the river until its natural course was blocked . 
Tunneling began at the lower portals of the Nevada tunnels in May 1931 . Shortly afterward , wrk began on 2 similar tunnels in the Arizona canyon wall . In March 1932 , wwrkbegan on lining the tunnels with concrete . First da base , or invert , was poured . Gantry cranes , running on rails thru tdaentire length of each tunnel was used to place the concrete . The sidewalls was poured next . Movable sections of steel forms was used fir the sidewalls . Finally , using pneumatic guns , the overheads was filled in . The concrete lining 3 feet ( 1 m ) thick , reducing the finished tunnel diameter to 50 ft ( 15 m ) . Thdaiver was diverted into the two Arizona tunnels on November 13 , 1932 ; the Nevada tunnels was kept in reserve fir high water . dis was done by exploding a temporary cofferdam protecting the Arizona tunnels while at the same time dumping rubble into thedaver until its natural course was blocked .