Sorry.
While reading this forum clarifies some things, others become more uncertain.
Situation: Covenant on an island, with an Aegis of the Hearth.
An enemy decides to magically flood the river, and drown the Covenant.
Under which circumstances does the AotH protect / not protect against the flood?
Asking questions is okay, that's why we have a forum!
It's going to basically depend on whether the water reaching the covenant is magical or not. Deluge of Rushing and Dashing, for example, floods a stream. As you're adding a ton of water to the stream for duration Conc, the aegis will stop the flooding at it's edge (unless the spell penetrates). Wizard's Flood, however, is a Creo momentary ritual. Since the extra water isn't natural once crated, the aegis will be unable to stop it. Rego will likewise tend to cross the aegis if all it does is temporarily propel existing water - e.g. Waves of Drowning and Smashing, for example.
As temprobe stated, with a caveat.
For rego, if you are moving the water with the spell it is stopped an the water falls outside the boundary (possibly inundating either way, but with little to no destructive force).
If you are just putting it into motion, something like a "Flood of Vilano", then the water would be able to cross the Aegis, but I don't know how much destructive power it would keep under these circumstances since you can't "toss" water in the same way you toss a stone.
You could also build a dam (CrTe, CrHe...?) downstream of the covenant in the hope of making the water level rise, but I think this isn't what you are thinking about.
I didn't think of dam.
If the river flow was great enough, and the valley was deep enough... And the dam created after dark so that the defenders didn't see it before the fires are extinguished by rising water.
I am currently considering various tactics and strategies that can be used by the attacker in a Wizard War. I am presuming his first tactic is to get his opponent, and as many of his defenders out from the protection AotH and associated buildings. And also presuming that the attacker cannot breach the AotH
The AotH should protect against turning the ground to mud and sinking the Covenant site under the river.
Bear in mind that, unless the attacker is at war with the entire covenant, doing shenanigans like trying to flood the covenant would be a hermetic crime resulting in forfeit immunity allowing anyone in the covenant to legally slay him.
To reduce distraction, let us assume the "Covenant " is merely the sanctum of a songle mage.
Magically created water will be resisted.
Magically moved water will be resisted.
Note, though, that mundane water, one not animated by magic, will no longer be resisted.
So if, say, I take a lot of mundane water (say, a lake's content), and move it against the aegis, so that it's surrounded by a 2 meters high ring of water, once I release my spell, this water will behave normally, including falling inside the aegis. That's not very efficient, though.
What I would do is to either:
Don't forget that geography plays a role here.
If the water has little space to flood into, you don't need much of it to flood the covenant, and conversely.
One devious way you can use active magic to make a flood worse is to "coral" the water.
If, way, I use creo terram to surround the aegis with a stone wall and then pour mundane water inside it, the wall will physically stop the water, but the aegis won't, so the water will naturally flow where it can.
However, this may take some time, and is easily detected and defeated by the covenant's occupant. At worst, it should give them a warning, and you don't want this.
You could also "build" a ReAq "dam" further downstream and along river banks to cause flooding that would not need to penetrate. Moon duration should work just fine.