... that kind of pressure, .
Even thoughts of speed are tricky.
If we think of a flow that could cover, say horizontally, a distance of 30 feet before crashing to the ground, we (with conceptions out of D&D physics) might think of something frighteningly fast.
If we thought of a continual flow, round to round that, over 6 seconds, only delivered 30 gallons of water in a 1 foot wide flow, we might think of something dawdlingly slow.
Whatever happens, the maths don't add up.
The text describes that:
"Geyser" produces 30 gallons of water that gushes forth in a geyser 30 feet long ...
then it couldn't produce a continual flow, round to round, that was:
On the 1 foot wide idea, it might come in geyser spurts 3-4 foot long and 1 foot wide and reaching a distance of 30 feet before gravity pulls it down.
A 1-foot wide, continual, round-to-round, 5 gallons per second (30 gallons per round) flow would only have moved 3-4 feet in one round at a speed, as I work it out, of: 0.3579 mph (with a US-based gallon) or 0.43 mph (with an Imperial-based gallon).
However it works, the result is of a not-too-challenging DC 13 strength check (that a commoner has a 35% chance of passing which, if passed, would result in no damage and, only if failed, would result in a d4 of bludgeoning damage, less than from falling 10 ft. This costs the user their action on each round of use.
The image we may get of speed probably relates to an idea of a 30 foot, potentially horizontal, stream of water needing to move fast so as to cover the distance before gravity took effect. In six seconds, in rw physics, an object can fall 579.133 feet and reach a velocity of 131.62 mph. Water bombs, from a high altitude, if anything, might work. =D
..., how viable is it as an energy source? Could it power a water wheel, or even sustain a large enough reservoir to be usefully dammed? ...
It could certainly be used as an energy source if placed at a height and, in this case, the water could even generate speed.
An internally consistent text might either describe a:
"Geyser" that produces 30 gallons of water in gushes 1 foot wide that reach a 30 foot distance in a round.
or to describe a:
"Geyser" that produces 30 gallons of water that gushes forth in a geyser 30 feet long and less than 1 foot wide.
1 US gallon = 0.133681 cubic foot by volume,
(30 US gallons = 4.01 cubic foot by volume) while;
1 Imperial gallons = 0.160544186995376 cubic foot by volume,
(30 Imperial gallons = 4.82 cubic foot by volume).
Here are calculations on view of continual flow:
Let's "speed" the water up and push it through a circular nozzle,
the ratio of the areas of a square and a circle is 4:3.14159265359... or 1:0.78539816339 (= side length of a square, squared to 1/2 diameter * π squared).
30 US gallons in a 1 ft wide "jet" would "stretch" all of:
30*0.133681*0.78539816339
= 3.1497843564 (feet per six-second round, less than a grid square),
30 Imperial gallons in a 1 ft wide "jet" would "stretch"all of: 30*0.160544186995376 *0.78539816339
= 3.78273328827 (feet per six-second round),
The speed of a US gallon "geyser" would be:
3.1497843564/6 = 0.5249640594 feet per second (6.3 inches per second)
= 0.3579 mph = 0.576 km/h
The speed of an Imperial gallon "geyser" would be:
3.78273328827/6 = 0.63045554804 feet per second (7.57 inches per second)
= 0.43 mph = 0.69 km/h.