ISP LCL CTLR INVOLVED IN OPERROR GND INCIDENT WITH COMBINED CTL POS WITH OTHER CONFLICTING AIRBORNE TFC CAUSING A DISTR.

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ON AUG/THU/01; AT ISP ATCT; THERE WERE 3 CTLRS ON DUTY (ONE A PQ) BECAUSE OF 2 CTLRS TAKING SICK LEAVE. AT THE TIME IN QUESTION; I WAS WORKING THE #4 POS NOTED AND RWYS 15R AND 15L WERE ACTIVE. THE ONE OTHER CTLR IN THE CAB WAS WORKING FLT DATA; CLRNC DELIVERY AND WAS CTLR-IN-CHARGE. I WAS RESPONSIBLE FOR ALL MOVING ACFT AND VEHICLES ON THE ARPT. THERE WAS A MAJOR CONSTRUCTION OP ON THE ARPT AND A LOT OF VEHICLE ACTIVITY. I HAD 9 ACFT ON FREQ; 4 IN THE AIR AND 5 ON THE GND. THE DH8 WAS ON A R DOWNWIND FOR RWY 15R AND I WAS GIVING HIM THE CALLS FOR A CESSNA DOING A PHOTO-SHOOT TO THE NW AND A RH22 HELI INBOUND FROM THE N. ALTHOUGH THE DH8 WAS CLRED FOR RWY 15R; HE BECAME CONFUSED AND LINED UP FOR RWY 10. A BE90 HAD JUST CROSSED RWY 10 AT THE NUMBERS. A CESSNA WAS ABOUT 800 FT FROM ENTERING RWY 10 TO CROSS AT THE NUMBERS OF RWY 10; AND A B737 WAS 500 OR 600 FT FROM XING RWY 10 AT THE MIDDLE. BECAUSE OF THE POTENTIAL CONFLICT WITH A PA28 IN THE TOUCH AND GO PATTERN FOR RWY 15L AND MY ASSESSMENT THAT THERE WAS NO CHANCE OF A COLLISION WITH TAXIING ACFT; I CLRED THE DH8 TO LAND ON RWY 10. IF I HAD AN ATC WORKING GND CTL; THIS ALMOST SURELY WOULDN'T HAVE HAPPENED.