Narrative:

I was working local control [local] combined with tcic [tower controller in charge]. Aircraft X called me with 'the ATIS' requesting to depart fpxx (float pond) opposite direction. I had opposite direction traffic arriving; but I advised him that if he would be ready in about three minutes (I had a gap) I could accommodate that since I had another aircraft waiting to go opposite direction off of runway xy right as well. He advised that he would be ready in three minutes and I instructed him to advise when ready for departure at fpxx. He called ready and I advised him to remain outside the channel; traffic was landing opposite direction. He replied 'roger; aircraft X.' we don't typically tell aircraft to 'hold short' of the float pond since there is no designated hold lines or markings; so instead it's common to use 'remain outside of the channel'. Aircraft Y was on short final for fpx; he advised that there was traffic departing opposite direction. I looked down the float pond to find aircraft X airborne midfield off of fpxx just above the trees. I instructed him to 'turn left on course now.' I had intended to give him the wind and altimeter when clearing him for takeoff; but since I didn't get the chance; I never verified that he had the current ATIS. When I told him on frequency that he had been instructed to remain outside the channel; he was confused because I told him to be ready in three minutes and when he called ready he thought I had issued him a takeoff clearance. I did not notice his departure sooner because there is a known blind spot at the north end of the fp (the threshold for fpxx). I did not push for a read-back of my instruction for him to remain outside the channel either and no such read-back was obtained. The north end of the float pond is also known for intermittently poor radio coverage; so that may have been a factor in him hearing that he had been cleared for takeoff even though no transmissions that sounded like a landing/departure clearance were made during that time. The estimated lateral distance between aircraft Y & aircraft X when aircraft X turned to avoid was approximately 3;000 feet; aircraft Y was just coming up on taxiway B; aircraft X was just past mid-pond. Aircraft Y continued inbound for landing FP2; going around could have worsened the situation.you have my permission to share with concerned parties.the blind spot at fpxx should be addressed. I should have obtained a read-back for the pilot to remain outside the channel/hold short of fpxx (it would be helpful if there were markers for this).

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Original NASA ASRS Text

Title: Tower Controller reported a NMAC between opposite direction traffic.

Narrative: I was working LC [Local] combined with TCIC [Tower Controller in Charge]. Aircraft X called me with 'the ATIS' requesting to depart FPXX (float pond) opposite direction. I had opposite direction traffic arriving; but I advised him that if he would be ready in about three minutes (I had a gap) I could accommodate that since I had another aircraft waiting to go opposite direction off of Runway XY Right as well. He advised that he would be ready in three minutes and I instructed him to advise when ready for departure at FPXX. He called ready and I advised him to remain outside the channel; traffic was landing opposite direction. He replied 'Roger; Aircraft X.' We don't typically tell aircraft to 'hold short' of the float pond since there is no designated hold lines or markings; so instead it's common to use 'remain outside of the channel'. Aircraft Y was on short final for FPX; he advised that there was traffic departing opposite direction. I looked down the float pond to find Aircraft X airborne midfield off of FPXX just above the trees. I instructed him to 'turn left on course now.' I had intended to give him the wind and altimeter when clearing him for takeoff; but since I didn't get the chance; I never verified that he had the current ATIS. When I told him on frequency that he had been instructed to remain outside the channel; he was confused because I told him to be ready in three minutes and when he called ready he thought I had issued him a takeoff clearance. I did not notice his departure sooner because there is a known blind spot at the N end of the FP (the threshold for FPXX). I did not push for a read-back of my instruction for him to remain outside the channel either and no such read-back was obtained. The N end of the float pond is also known for intermittently poor radio coverage; so that may have been a factor in him hearing that he had been cleared for takeoff even though no transmissions that sounded like a landing/departure clearance were made during that time. The estimated lateral distance between Aircraft Y & Aircraft X when Aircraft X turned to avoid was approximately 3;000 feet; Aircraft Y was just coming up on Taxiway B; Aircraft X was just past mid-pond. Aircraft Y continued inbound for landing FP2; going around could have worsened the situation.You have my permission to share with concerned parties.The blind spot at FPXX should be addressed. I should have obtained a read-back for the pilot to remain outside the channel/hold short of FPXX (it would be helpful if there were markers for this).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.