Terrycloth notes stuff that I interpret as, "the first player has a huge advantage, instantly putting the second player on the defensive." I saw that too: the first player can get an orren in the river instantly, which is a very big deal.
Also, I've found crossing the river to be very awkward. If you put a piece in the river, you're pretty much begging to be forked, since the river itself is attacking the piece that's in it. Also, having an orren in the river is a huge advantage.
1 = hate the idea, 10 = love it.
Put a lake in the river, so that the four central squares of the board are water. That way the Sleeth can't zoom across the board from city to city in one turn.
Have the turn sequence be "Move, then muster" rather than "muster, than move". That stops the muster-a-sleeth-and-zoom-down-the-diagonal-to-win trick. It also means you can usually muster a piece in a turn if you like, using your move to clear out your town.
Allow mustering on the city or any square adjacent to it, instead of just in the city. In this version, mustering replaces moving a piece. This way you can pop a piece onto the board to defend your city at need, often.
Allowing mustering on or adjacent to the city and a move. This makes your home territory very dangerous to approach.
Say that orren can't capture on a river-to-river move. With this approach, an orren in the river is still wonderfully mobile, but doesn't quite rule the river.
Say that orren can't take each other in the river, but they can take other kinds of pieces. This lets both players have their orren in the river at the same time. This cuts down the first player's advantage in getting the orren to the river quickly. A river with two orren in this variation in it is an equal barrier to both players.
Say that World Tree people who can naturally fly can move over the river, or stop in (over) it without risk of drowning. This would be: sleeth, khtsoyis, zi ri. This makes physical sense, and makes the river much less of an obstacle. I'm not exactly sure how it helps gameplay at all.