Alpaca
In response to your how do you view different angles question. - you move the view from left to right or zoom in or out.

I see this the black bar bit on the vanilla map also and I need to check but I think its vanilla with the HGT in also.

I need to convince myself that both of these always occur at the same location but I'll try the heights thing although I don't see why it should be connected - and I'm seeing it quite a bit in relatively flat areas. My hunch is its memory although I've got 2 GB in my machine.

I haven't spotted the rivers thing in vanilla but the black bar definitely appears there.