First question is always: what is the intended purpose of this map? I observe that the federal land boundaries aren't masked by the in basin/out of basin gray boundary, suggesting that those boundaries are of greater importance than the basin boundary.
The basin boundary edge feels a little diffuse; perhaps a hard line (green?) along that boundary would let you delineate it more clearly?
Does.the masked area change the hue and saturation? You can probably get the basin area to pop more without losing information by dropping the saturation for the outside.