It does happen, as I have said on another map. I even checked and wikipedia has found 7 instances where this happens in real life. Worldwide. That said, it's quite rare.

Easy, I would cut the river, that connects the lake in the south with the river system north west of the mountains. From the many river outlets, this looks like the mountains there are quite massive and it would require the water to flow upwards quite a bit to flow over those mountains. Sure, with enough water pressure, the water could break its way through, but this pressure will not happen, since it is connected to the ocean already on both sides.

The Plateau think is different. You looked at a river delta, I guess.
The problems are, that
1. This does not scale up. The Nile delta already is almost impossibly huge. If it was not real, it would not be realistic. Yours is magnitudes bigger.
2. It really just works at the coast or really big lakes (where the sediments have time to set to the ground and are not simply washed down the waterfall. ).

At least remove the first disjunction on the top plateau, maybe a few more. One or two can fall under the "it's unlikely but possible" but when it's too many it looks weired.