[Marileangorifurnimaluim]

Telchar's explanation, that the dwarves were able to flood the valley seems likely. Note that Thorin employed this very technique at the Lonely Mountain in The Hobbit.

I doubt that the West Gate was built with such a mechanism in mind, given the peaceful times in Hollin. More likely Balin himself ordered it, remembering Thorin's method of defending against greater numbers. He was there after all, and witnessed its effectiveness. Likely they were preventing attack and orc reinforcements from the west gate, hoping to defend the east gate, which was also their escape back to the Lonely Mountain. What was to the west for them - Rivendell? Hobbits?

A hurried job with little knowledge of the current denizens of Moria could have had some unintended bad results. Were there caves perhaps, or mine shafts in the valley, leading to the depths of Moria, that normally weren't flooded?

I can see it now.

The dwarves dam the river. But nothing happens, the water drains first into caves, mineshafts and unknown pits in Moria.

Their plan apparently failing, orc reinforcements enter the west gate, and the dwarves are pushed back to the greater halls of Moria.

Then the dwarves are attacked with fury on the other side at the east gates. After the fall of Balin the dwarves lose heart, and are forced to defend themselves to the east.

The dwarves hold out, defeat the lesser number of orcs from the west (the main attack fell at the east gate) and send dwarves to hold the west gate, just in case they can't win back the east.

They return to find that their plan has belatedly succeeded. Now trapping them.

The denizen of the deep, the Watcher in the Water, like any predator would naturally swim up the caves/mine shaft to roam its new territory in search of prey. Obviously with those tentacles and stealth it relies on grabbing prey as it stops to drink from its pool. Probably the dwarves flooded its usual hunting grounds. With the Watcher the dwarves could not un-dam the river.

It would snack on a dwarf or two, before those who escaped could return to the interim Lord of Moria, to tell him the bad news that the west gate was closed to them: they had to win back the East Gate or die. Clearly they no longer had the numbers to go against the main assault of the orcs, and there was no escape. So they did an Alamo-esque last stand in the Hall of Records.

As for Gandalf not knowing the river was dammed, that is clearly the result of the lack of satellite imaging. [img]smilies/wink.gif[/img] Seriously though, in wilderness areas even today people have to go to a place to see the trail conditions.

-Maril