General aviation pilot reported a NMAC with a helicopter approaching from the opposite direction while flying low level through canyons.

Narrative

Pilot of Aircraft X was executing a low level route from north to south over an unpopulated area at 500' AGL. The route is located in a canyon with multiple S turns and descending terrain that starts around 4k to 5k' msl in the north to 800' msl at the south portion. Near the end of the route; the Aircraft X pilot made a right hand turn in the canyon and visually saw Aircraft Y proceeding north bound (in the opposite direction) 3500' lateral separation and 500' above Aircraft X's altitude. The helo was descending into the canyon from the ridge to the west at a relatively rapid rate and the vertical separation was decreasing towards 300'. Based on the decreasing lateral and vertical separation and the helicopter being established above Aircraft X; the Aircraft X pilot decided to descend to avoid a flight path conflict. Aircraft X and Aircraft Y passed within 200' lateral separation with Aircraft Y gaining visual and climbing up and to the left at approximately 800' of lateral separation. Aircraft X aborted the remainder of the low level in the case there was another helicopter in the vicinity. No damage occurred to either aircraft or crew. Both pilots found each other on the local CTAF frequency and Aircraft Y stated that they would be operating in the area and Aircraft X agreed to remain clear of the route while they were operating in the canyon. Fix: Both pilots failed to make an advisory call about their location and intention of flying the low level route. While this call is not mandatory and flight regulations were being followed by both pilots; it would have increased both pilot's situational awareness of the other aircraft's location and intentions and potentially alleviated the close pass and requirement to take evasive action at low altitude.