Impassable rhymes with:
passable, passable, accessible, addressable, admissible, collapsible, compensable, comprehensible, crucible, defensible, dispensable, embraceable, enforceable, erasable, fixable, flexible, forcible, immiscible, impossible, inaccessible, inadmissible, incompressible, indefensible, indispensable, inexplicable, inflexible, insensible, invincible, irreplaceable, irrepressible, irresponsible, irreversible, leasable, miscible, nontaxable, noticeable, ostensible, passable, peaceable, permissible, possible, reducible, replaceable, reprehensible, responsible, reversible, rosabelle, sensible, serviceable, taxable, traceable, unenforceable, unserviceable, untraceable
Impassable sounds like:
impassively, impeachable, impeccable, impeccably, impossible, impossibly, infeasible, invisible