When floating helplessly down a river, it is one of the universe’s favorite running gag to insert a waterfall in.
When floating helplessly down a lava river, it’s that sort of sense of humor that make you want to punch God in the nose.
Real Time: 1 minute to figure out what to do. This can be extended by at least two PCs paddling, or somehow keeping the boat from moving 6 squares/turn.
Applicable skills: Strength check, 1 square per 5, endurance each round, athletics. Dungeoneering to get them deliberately snagged on a rock (perception can add a +1 bonus).
The Lava Falls! One of the great Wonders! A drop that goes down hundreds of thousands of feet, to drop you directly into the Underdark. The drop goes on for so long that it has its own ecosystems. This, of course, means that the party is attacked by fire mephits on the way down. If mephits are too weak, some other demon-y primordial kind of critter. And fire. Lots of fire.
More important to note is that everything appears to slow down during the drop. That’s because gravity weakens toward the middle, until it’s a complete free fall at that point. Only a very high Nature check will catch this. The immediate advantage is the ability to collect some rare specimens of floating plants, (worth thousands to the right folks), and reagents (for fire spells, flight/TK, shadow, primal) for rituals and powers.
Toward the bottom of the fall (takes a week, do they have food?) speed begins to pick up. There’s a giant lava pool waiting for them. How do they survive? A huge Athletic jump from the boat will only mean probably lethal damage, on the hard, hard ground. The boat will get crushed and destroyed by the lava pool. The unknown part of all this is that the passage away from theunderdark is through that pool. Before any of this, see the Primal Fire chapter