ACR SF34 FLC; CLRED TO LAND BEHIND DASH 7; INITIATES A GAR DUE TO ACFT ON RWY. AT TIME OF GAR; TWR HAS CLRED A B737 FOR DEP FROM A XING RWY. THE SF34 PIC OBSERVES THE B737 CLBING IN CONFLICT WITH HIS ACFT AND INITIATES AN EVASIVE TURN. NEITHER THE TWR NOR THE B737 APPARENTLY SEE WHAT HAS TAKEN PLACE. FO STATES IF IT WERE NOT FOR PIC TAKING EVASIVE ACTION; A COLLISION SEEMED OBVIOUS.

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SIMULTANEOUS OPS ON RWYS 9R&L AND RWY 12 WERE IN OP. WE WERE ON VECTORS WHEN CLRED FOR THE VISUAL RWY 9R; REF TO OUR TFC; A DASH 7; AND HOLD SHORT OF RWY 12. WE WERE THEN ASKED TO SLOW TO FINAL APCH SPD FOR SPACING. I IMMEDIATELY CONFIGURED AND SLOWED TO FINAL APCH SPD. THE DASH 7 LANDED AND TOOK ITS TIME ON ROLLOUT AND TAXIING CLR. I DO NOT RECALL ANY COM TO THE DASH 7 PERTAINING TO EXPEDITIOUS CLRING OR SUGGESTING AN INTXN. AT SOME TIME DURING THIS APCH; THE TWR LAUNCHED A B737 FROM RWY 12. WE WERE ON GS AND APPROX OVER THE DISPLACED THRESHOLD WHEN WE DECIDED TO GAR. WHILE CLBING STRAIGHT AHEAD AND CONFIGURING THE AIRPLANE; THE CAPT SAW THE B737 CONVERGING OUT OF HIS L SIDE WINDOW. I DID NOT HEAR ANY REF MADE TO ITS POS FROM THE TWR; NOR DID I HEAR ANY REF MADE OF US TO HIM. THE CAPT IMMEDIATELY GRABBED THE YOKE AND APPLIED FULL R AILERON UNTIL WE WERE IN APPROX A 60 DEG BANK. WHEN WE WERE CLR OF THE CONFLICT; WE WERE VECTORED AROUND AND LANDED ON RWY 9R. WITH THE CAPT'S ESTIMATE OF THE DISTANCE OF THE CONVERGING B737 AND ITS APPARENT NONEVASIVE TURN; AND ABSOLUTELY NO HELP FROM THE TWR; THERE WOULD HAVE BEEN DISASTER.