however, the rumblings continued. The noise was so noticeable that a worker in the employee dining room, located near the power plant, reported that it "sounded like the barrages that he had experienced in Vietnam". The engineers soon closed the spillways for an inspection. Cavitation damage found Inspection crews were lowered down the spillway tunnels in a small cart to assess damage. They found that cavitation, a known risk associated with tunnel spillways, had severely damaged and eroded the -thick concrete tunnel lining. In some locations the cavitation had exposed the soft sandstone. The tunnels could not be kept closed as more rain fell in the Colorado River Basin and the reservoir continued to rise. Reclamation reopened the left spillway, allowing per second; the right, which had suffered worse damage, was only permitted to carry per second. Emergency flashboards installed As the reservoir level increased, plywood flashboards were installed on top of the spillway gates. They allowed reservoir water to rise without increasing releases. Tunnels damaged further The left tunnel, however, was suffering further damage, described as a "surging, boiling flow that filled the portal". The tunnel had formed a hydraulic jump as it was turned into a "pressure conduit" by the surging flow of water. The interior erosion threatened to collapse the tunnel. Engineers knew that increasing the flow would decrease the turbulence but would further damage to the tunnel walls. To increase the reservoir's capacity further, engineers began an emergency effort to add to the height of the spillway gates, avoiding increasing the rate of water releases. Diversion tunnel threatened The force of the water at the left tunnel eroded concrete near the diversion plug, a device that blocked the diversion tunnel used during construction. The Bureau of Reclamation was concerned that the water would eventually erode the diversion plug altogether, creating a connection to the reservoir floor. This uncontrolled spillage would cause the reservoir to drain, creating a catastrophe. As the discharges from Glen Canyon Dam reached almost per second, with per second racing through the left spillway, the of flood storage space behind Hoover Dam downstream was exhausted. Lake Powell reaches peak level On July 15, 1983, Lake Powell reached its peak level, about from where engineers thought they'd lose control. The water held steady for a few days and then gradually declined. Inflows to Lake Powell topped per second, while releases from Glen Canyon Dam