Put the dye packet in a mesh bag with several large (baseball sized rocks) and lower to the bottom where you think the leak is .Let it sit for a couple mins. Then jerk the bag up and down a few feet. The rocks will help mash and disperse the dye and the leak should pull it in and it will come out down stream. Another trick is to mix some dye and water and put into large balloons , tie 2 strings to the neck of the balloons and lower to bottom near the leak and pull the strings in opp. directions to break the neck and release the dye. You can use a sharp stick to break the balloon if the water is not to deep.

Good luck and keep us posted. We did this with a pond that had a leak through a permeable sand. Once located we made a batch of drilling mud (bentonite used in well drilling) and because it is heaver than water we poured it down a 6 in pvc pipe that went to the leak area on the pond bottom. It worked well. Don't know if it would work on a leak the size of yours.