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In an underwater streamer discharge, plasma channels are created and propagate in water. After the discharge, the ionized gas channels gradually cool down and decay. In this paper, a time-resolved study on the decay process of the gas channels induced by pulsed positive streamer discharge in water is performed by using a high-speed camera system. The gas channels created by the streamer discharge in water have many branches. Some of them grow from the tip of the point anode, forming main branches. From the main branch, some subbranches develop. The subbranch dilapidates rapidly into microbubbles, while the main branch abbreviates toward the root of the branch. A bubble that formed at the tip of the anode connects with the main branch. The main branch does not completely disappear until 80 μs after the discharge. When the main branch disappeared, the bubble at the tip of the anode becomes a perfect sphere and approximately reaches its maximum radius. Then, the bubble starts to collapse, holding its spherical shape, and reaches its minimum radius at about 170 μs after the discharge.