The Honjo area was surrounded by dikes for reclamation work and separated from Lake Nakaumi in 1981. The work has been frozen for 17 years. The survey for the decision whether the work will be continued or not have been carried out from 1997 to 1998．Tidal flow pipes were constructed for the evaluation of fishery promotion between Honjo area and Hokubushosuio channel in March 1998.
Water quality of the Honjo area was surveyed from May 1997 to November 1998 at 7 sites in the area including 1 site in the Seibushosuiro and 1 site in the Lake Nakaumi. In order to evaluate the effect of tidal flow pipes, water quality data before and after was compared. No change was observed except one station near the pipes. A little change was observed at some stations.
Water was well mixed vertically and horizontally in Honjo area, so no strong halocline condition was founded. Total nitrogen concentration in Honjo area was lower than in Lake Nakaumi, but total phosphorus concentratios were almost smilar in both area. So it was considered that Honjo area has ability to purity the water of Lake Nakaumi as to nitrogen under the exchange of water between both area.