Oboke and Koboke
The Yoshino River which forms the Oboke and Koboke Gorges flows from Kochi Prefecture in the south up to the town of Ikeda to the north. Oboke is a bit larger and near the prefecture border, and Koboke is just to the north (its hard to really tell where one gorge ends and the other begins, as they more or less merge together.) The main access road of Rt 32 follows the river and runs entirely through the gorges from Ikeda to Kochi, and the JR Dosan train line also follows this route.
For lots of detailed information in English about Oboke, Koboke, and the neighboring Iya Valley, check the extensive IyaTime website, which also has lots of interactive maps and access guides as well as details regarding just about every place and provider in the area.
It is recommended to travel in this area by car. If you get there by train, you'll have a long walk along a noisy road without sidewalks. It is also hard to find entry points for hiking trails without a car. Unfortunately, the nearest car rentals are located quite far away: Marugame, Takamatsu, and Tokushima.
National Highway 32 winds along the Yoshino River through the pristine valley. Coming from Osaka, Kobe or Tokushima, take the expressway and exit at Ikeda. Then follow route 32 towards Kochi for about 20 kilometers to Oboke.
A tunnel also connects Oboke to the western Iya Valley.
Buses go to/from the main Awa-Ikeda Bus Terminal in Ikeda through Oboke and Koboke and then on to the Iya Valley. Several additional buses go from Oboke Station to the the Iya Valley on weekends/holidays and less so during the week. For complete bus schedules in English through the area, check the IyaTime Bus Timetables.
See & Do
There's one big reason to visit: the rapids rushing through the gorge below. There are observation platforms on the highway and near the train stations if you're content to gaze from a distance, but more daring types will venture down onto the Yoshino River for a sightseeing cruise (¥2000 for a one-hour loop), and more adventurous types yet will try their hand at white-water rafting in the class 4 rapids. The rapids are Japan's biggest and most exciting, with amazing rock formations and crystal clear waters.
There are about a dozen rafting companies in the area, and most offer full and half-day rafting tours in English, and some also offer canyoning and other water sports. For a complete list in English of the rafting outfitters around here, check about "See and Do" section of the IyaTime website.
If you survive the experience without developing acute hydrophobia, both Oboke and Koboke have onsen hot springs offering more bubbling water to soak in.
Ikeda is the closest town with a real supermarket.
Eat & Drink
The local specialties are ayu and amego, delicious river fish salted and barbecued on a stick.
Sleeping in the area is useful for exploring the Iya Valley and surrounding towns, but is a bit far of the eastern end of the valley. If looking for hiking opportunities, lodging closer to Mt Tsurugi might be a good idea. For a complete list in English of places to sleep or camp in the Oboke and Koboke area, check about "Places to Stay" on the IyaTime website.