The Río Grande de Orosí (REE oh GRAHN deh theh oh ROW see) is the primary headwater source for the Reventazón, the most heavily used whitewater stream in Costa Rica. It is a pristine stream which flows from a watershed that is maintained in virgen forest in order to protect the water supplies of San José. In the upper reaches, much of the streamflow is diverted from the stream bed for that municipal water system and to generate electricity. As a result, the top sections are rarely runnable. Just above Cachi Reservoir, water quality drops off abruptly with the inflow of the urban runoff and sewage from the city of Cartago.
Grande de Orosi Section: Dos Amigos to Tapanti
Degree of Difficulty: V,VI
Gradient: 166 feet per mile (31 m/km), 3@ 204 ft/mi (5 @ 39 m/km)
Length: 3.3 miles (5.3 km)
Put-in Elevation: 4854 feet (1480 m)
Take-out Elevation: 3900 feet (1190 m)
Drainage Area: 100 square miles (257 km2) at take-out
Average Discharge: highly variable due to diversion
Season: July through December, intermittently.
During the high water season of October, 1984, Kevin Knussmann, Fernando Castañeda, Tom Wise, and Oswaldo Trejos attempted the uppermost section of the OrosÌ, putting in at road's end above TapantÌ, at the mouth of the Río Dos Amigos. The river flows through a gorge of incredible beauty in this uppermost section, but the gradient is over 200 feet per mile (39 m/km) in the first three miles (5 km), and the difficulty of this section is even greater than the gradient implies.
Peter Spiers has run this section several times; his group has now run all but one of the drops in the gorge. Peter rates 18-20 of the drops as Class V. There are numerous rapids and waterfalls that should be scouted and possibly portaged in the Refugio TapantÌ section, including a double drop of over fifty feet (15 m) that requires a long rope and steady nerves to portage, for the canyon walls are steep and very slippery.
The first descent group managed to run drops of 8', 13', and 16' (2.5, 4, and 5 m) but the 8' (2.5 m) ledge breaks onto poorly padded rocks. In addition to these "small" drops, the group encountered a 30 foot (9 m) waterfall whose plunge pool quickly leads into a second drop of 20 feet (6 m). The group managed to portage the double drop with considerable difficulty and reported that even that would have been impossible with only a little more water. They finally ended the trip at Palomo after dark, having averaged only one mile per hour.
This section of river is extremely hazardous; rising water levels could cause disastrous results. The Tapantí section is rarely runnable because much of its flow is diverted from the channel above the put-in to supply the water for metropolitan San José. In addition to these difficulties, the Costa Rican government now charges an access fee of $15 per person for entry into the Tapantí reserve.
Grande de Orosi Section: Tapanti to Palomo
Degree of Difficulty: V,VI
Gradient: 105 feet per mile (20 m/km), 0.4 @ 170 ft/mi (32 m/km)
Length: 3.3 miles (5.3 km)
Put-in Elevation: 3900 feet (1190 m)
Take-out Elevation: 3575 feet (1090 m)
Drainage Area: 144 square miles (371 km2) at take-out
Average Discharge: 1230 cfs (35 cms)
Season: July through December, intermittently.
On a map, this looks like an ideal section to paddle; it has easy access, a steep but manageable gradient of 105 feet per mile (20 m/km), and the sections visible from the road are obviously runnable. Beyond the first bend, however, the river enters a small gorge. The gradient quickly increases to 120, then 173 feet per mile (23; 33 m/km) as the drops become large bedrock ledges. The first significant rapid on this section begins where the river bends to the right into a maze of boulders. Known as Dinosaur Rapid, most parties have portaged it along the right side. This rapid is only 25 yards long, but it is followed by Dinosaur's Nostril, a menacing rock jumble with a seriously undercut rock. Both drops have now been run, but they are both Class V or VI, depending on water levels.
The river now settles down to mere Class IV-V, with many radical but runnable drops until the current is divided by an island. The right side is marginally runnable (Class V+). Finally, the difficulty of the rapids decreases considerably, with only one difficult rapid, another Class VI, after the island. Two hundred yards (180 m) past the island is a hot spring area on the left that is useful for soothing sore backs and shoulders that have been abused by insane rapids or portages. There are no more difficult rapids until the junction with Río Macho.
Ecotourism and Nature 
