Volcanic ash and cinders blanket the Río Cajón basin, which lies downwind from Volcán Hudson (1369 m). After laying dormant for twenty years, Hudson erupted in August 1991 wreaking considerable property damage and environmental change in Chile Chico and western Argentina. Meter-thick ash deposits buried a 25 km section of the Carretera Austral.
Spend the time, roughly six hours, to thoroughly scout the entire run before committing to a descent. The ash fall and resultant carnage in the forest have knocked many trees into the river. They add considerably to the already high level of danger on this run.
At the put-in, the Cajón cuts a braided channel below glacial-covered peaks. After a few kilometers, the river enters a narrow 30 m-deep class II basalt canyon, which resembles a half-pipe draped with hanging vegetation. When the geology changes to granitic basement rock, look for a sawmill on the left that marks the beginning of the tough drops. Less skilled boaters should take out here. Downstream, you must negotiate several log strainers, sieves, and class IV rapids en route to a portage around a big log jam. Countless trees have piled up in this narrow cleft to form a massive 6 m-high obstruction. Do not miss the last chance micro-eddy just above this death trap where the swift current piles into the trees. In January 1997, a German team lost a boat in the big log jam after someone swam upstream. Apparently, they recovered it after a few hours of probing.
A 100 m-deep, 3 m-wide, unscoutable slot canyon lies not far downstream. Drag your boat up to the rim, hike downstream through the trees on the pumice, and put in immediately below the end of the slot canyon where you can paddle back upstream through this dark, narrow chasm.
Below here you will encounter a vertical-walled canyon with wall-to-wall class V rapids, many blind turns through large boulders, and extremely difficult scouting at river level. Do not attempt this section unless you are up for the challenge.
From Villa Cerro Castillo, 100 km south of Coihaique, drive west 20 km on the Carretera Austral heading up the broad Ibañez valley until the road leaves the main river and follows the Cajón. Look for a decent take-out shortly thereafter 375 meters elevation. Scout the lower canyon with binoculars on your way upstream. Put in at an old corral approximately 11 km upstream at 540 m elevation
This class V+ stretch is 11 km long with average gradient of 15 mpk or 80 FPM and has flow of 600 CFS in spring and summer. Expect 5-6 hours to complete the run and have 3-4 portages.
Topo maps are titled Chile Chico • Cerro Sin Nombre, Cerro Hudson