Hydrolic Hijinks

So apparently in "Rings of Power" an assault is launched across a riverbed after catapults of dubious design and curiously striking power knock rocks off a cliff and dam a river. Knocking rocks into a river usually results in harrowing rapids, and not a dam.

Soap Creek Rapids, Badger Creek Rapids, Crystal Creek Rapids, Lava Falls. Nearly all of the time, the creeks that plunge down the ravines of the Grand Canyon will barely float a walnut shell, but the flash floods resulting from a desert downpour can dislodge boulders as big as a jitney bus. Tumbled by gravity, the boulders carom into the main river and sit there, creating a dam, which doesn’t so much stop the river as make it mad.

…

At Lava Falls, where huge chunks of basalt dumped in the main river create a thirty-foot drop, waves at flood stage were as high as three-story houses. There was a cycling wave at the bottom that, every few seconds, would burst apart with the retort of a sixteen-inch gun, drenching anyone on either bank of the river—two hundred feet apart. To run Lava Falls today, in a thirty-foot Hypalon raft, wrapped in a Mae West life jacket, vaguely secure in the knowledge that a rescue helicopter sits on the canyon rim, is a lesson in panic.

— Reisner, Marc. Cadillac desert. Penguin, 1993.

Blocking a river takes a bit more, though is not impossible. In particular one needs favorable geology, or must summon glaciers, the kind that trundle down from Canada every so often. Favorable geology would include the Columbia River along the Oregon-Washington state border; the strata is aligned such that the Oregon side has cliffs and waterfalls, while the Washington side is like a skid row designed to bring material to the riverbed. Hydrology is also important; doubling the height of the water table usually halves the angle necessary for a stable slope, and a lack of drainage that keeps the water around may also be necessary: a layer of clay, or something hard enough. Specific triggers for landslide are rather unknown, beyond "well, it had rained a lot recently", so humans being able to cause a sufficient landslide on demand seems unlikely.

The "Bridge of the Gods" is a native account of a landslide that blocked the Columbia River. This involved something like eight square kilometers of material coming off a mountain. An invasion of glaciers can result in such things as Glacial Lake Missoula. These are rare events (at least in terms of human history) so the odds of a landslide happening at just the right time for an army to make use of it seems rather unlikely. Sort of like a duck falling out of the sky into your cooking pot. Possible, yes. Likely, no.

Another problem is how long the dam persists; all dams are temporary. In the "Bridge of the Gods" one estimate is that the lake took about six months to fill. Another point is that the salmon did not go extinct, so with rather large error bars one might put the shortest at "a few months or maybe less" as a lake forms and then catastrophically fails, while the longest might be upwards of two years or maybe more if the salmon can do their business downstream. More likely might be a few months to a year of blockage, and sufficient flow is established after a year or two for the salmon to get where they need to go. Maybe a run or two of salmon is lost. The dam in this case could fail "slowly, then all at once" or just slowly as the river nibbles away at it. A few months is enough for a quick siege, especially if the town is undefended along the river and the commander rolls the dice on a quick assault. Anything longer risks playing chicken with a "Fist of the Gods" from upstream. Also the details of the riverbed would be quite important: is it a muddy bog, or easier to cross bedrock?

So if you were doing storytelling, one might imagine a scout who witnesses the landslide, and rides like mad back to camp. The camp was opposite the town as a show of force; they did not expect to attack across the impassable river. The commander, who maybe has been established to know something about hydrology or engineering then estimates when the river will dry up, and thus prepares for a quick assault depending on how the riverbed turns out, how alert the townsfolk are, etc. This very much ties into luck, so the commander could be seen as lucky, at least until some point later in the story when they fall out of favor with the gods and all their plans come to naught.

(Someone who is better at hydrology than me should estimate how long a river would take to dry up and whether a horse riding back to camp could beat that. Wouldn't everyone notice a mountain falling into the river? Not really, there were people hiking quite near to Mount St. Helens unaware it had erupted—the clouds must be due to a forest fire?)