Hopefully that should be enough to get the water flowing strongly - if it looks like it's getting to a dangerous level he'll stop before that. He'll watch the flow for a bit (if it's not getting dangerous), then retire outside to wait for the flow to start diminishing again.
The water flow doesn't seem dangerous in itself--that is, it's not threatening to flood the passageway. However, after the second rock is removed, the pile begins to quiver from the weight of the water passing over and through it--it looks like it could easily give way.
Gregroius will leave it at that, then, and retire outside the caves (at a brisk pace, but not quite running unless he hears things start to go over behind him).
Where is he at this stage, where's the nearest way out of the water's path and how far away is the water? He'd been casting from a bit of a distance, and had already been leaving the cave by the time the blockage went, so hopefully he's got a bit of warning (and could hear the blockage collapsing?)
OK, let's back up a bit. The narrow quarry that the workers dug along the underground stream runs from well outside the clearing where the covenant is sited. Remember, it's taken the workers two years of off-and-on digging to get this far. Were you under the impression that the passage was much shorter? If so, we can go back and change what Gregorius did.
Yes, I'd been assuming it wasn't massively long - 15 - 20m say, and a decent width as they'd been working with it. Rereading the arrival thread, it's 100 paces just from the river to the treeline (plus a distance enough for the trees to count as "woods" beyond that to get to the covenant), so rather further than I'd thought. Fine. The masons are forgiven for taking two years to cut through.
Let's just run with it.
Gregorius swears briefly when he hears the water coming down the tunnel behind him, then begins casting. He's aiming to fill the tunnel behind him with a large plug of clay.
[Base 1 (create clay) + 1 Touch + 1 diameter + 1 size = Base 4. Casting total is (Cr 0 + Te 7 + Sta 1 + Aura 5(?) + die 8 )/2 = 8 + Aura/2. Should work. If possible, he'll have sited the block just before a narrowing of the quarry, but he's probably not in a position to be choosy.]
Then he'll hoof it down the passageway, feeling very glad he got everyone else out first.
That's probably not going to hold very long, but it at least gives Gregorius time to get out.
I'll go ahead and tell you he's not going to be doing very much for a bit--it takes hours before the flow finally starts to abate, moving toward normal. Gregorius suspects that the smashed lab he found has been at least partially flooded in the process, and so it's a good thing the magi grabbed those notes when they did.
Gregorius watches the flow for a little while, then when it shows no immediate sign of abating, sends someone to bring him some food.
Once he's eaten and the flow's abated, he'll send word to Fray (assuming he's wandered off at some point), then heads back into the tunnel to inspect the blockage. If necessary (and there's nothing obvious to deal with first), he'll get some of the quarryers to clear the remains of it so he can get past.
Yes, he's really going to need to get the pile cleared--at the moment, it's still partially blocking the passage, which results in flooding up the passageway (only a couple of feet deep, but still, Gregorius and Fray probably don't want to walk up there if they can help it).
Once that's out of the way, though, the magi find that the passageway continues to follow the natural stream course. It winds a bit, and slopes gently upward, finally ending at the base of a waterfall: a steep stairway has been cut into the rock face here, ascending to the source of the waterfall. Just below the waterfall, the stream branches, but the water in the branch is stagnant, and the passage in that direction is still partially flooded, with the water getting deeper as it descends.
"Yes, certainly. I think some gills will do the trick nicely. We can look now if you prefer. It is achievable now.
If we find the passages too small to pass or are blocked I may have to transform into a small swimmer of some sort. Oddly I think an Otter would be an ideal choice for most circumstances, but they would be downright terrible if trouble occurred."
Fray will spont cast a Gills of the Fish style spell, to give himself additional gills to breath water. Not using the default "lungs of the fish" from the core book as it is an Auram spell. Note: He'll offer the same to Gregorius, and not at all offended if he says no.
On himself I figure it's effect level 4. On another person its effect level 5.
Effect: MuCo/An 4
Base 2 to add a minor ability, Animal Req, R: Per, D: Sun +2, T:Ind
and cast on another it is R:Touch +1
CT for his spell is 51. Mu 21, (An/Co 11)x2 for magical focus, Int 3, Aura 5
CT for the spell on another is 40. Mu 21, An/Co 11, Int 3, Aura 5
[I've never been convinced by the idea that being able to breathe underwater counts as a "minor" ability - I'd add a couple of magnitudes to the base - maybe three. Fray should still be able to cast it regardless, though. Also, is there something particularly problematic about Auram I'm missing, beyond that you have a score of 0 in it (and also Aquam)?]
Gregorius will accept the spell, lowering his Parma if Fray thinks it necessary (which it presumably will be if we're boosting the base).
Then he'll wade back into the water. Then he realises that he's not going to be able to see anything without light, get back to a point where he can cast normally, and cast the following spell:
[CrIg Base (max he can manage) + 1 Touch + 2 sun + 1 Rego Requisite = 4 + Base. This should create a globe of light which exists for sun duration, and which he can move by concentrating on. Casting total (Cr 0 + Ig 0 + Sta 1 + Aura 5 + W&G 2 + die 2x3)/2 = 7. So base is 1, i.e, equivalent to moonlight. Still, he has eyes of the cat.]
I tend to agree about the "minor" thing, but I'll accept that changing the person to fish rather than water to air is simply a different way to skin the cat.
Once he manages to get down far enough, Gregorius finds another pile of rocks blocking the passage, similar to the first one, and with more or less the same effect, though on this side, it's hard to be sure if any water at all is flowing through.