I kind of like the idea of a lake in that area. I have added one, and am posting the results.
The bay that was formed was by an unnatural (magically induced) earthquake. It was during the age that the gods still walked the land, and during a war, a star from the heavens was called down (meteor), and it struck in the mountains here, which cracked the earth and shook the world.
The area between the north and southern mountains is very much like a high plateau similar to what could be found in the western U.S. between the Rocky Mountains and the Sierra Nevadas. It slowly rises from the east to the west.
The mountains that are in the east are only about 2-5,000 feet tall, where as the mountains on the west (just off of the map) are about 10 - 25,000 feet tall.
The northern mountains are in the 8-14,000 range, while the southern ones are in the 6-10,000 range.
The area where the two rivers drain from quickly drop from an average height of 1,000 feet to feed through two canyons (fracture lines) into the bay. The mountains on the south side of the bay were thrust there by the impact of the meteor, and they are fairly steep, but only run from about 2-7,000 feet in height.
Without doing a height scale map, it is hard to show how the land drops along the two gaps, but I am not very good at doing the height maps, and am too impatient to work at it! I will try to do that after I get the first set of maps done.