Sweeping the streambed of Kenrokuen garden

I encountered a rare scene at Kenrokuen garden when guiding tourists overseas. Gardeners were  sweeping a streambed on March 26, 2012. They used bamboo brooms. Administrative office of the garden says it’s for removing algae and dirt on pebbles. Stream water comes 11 km upstream of the Saigawa river through tunnel and open canal. That’s why Alga grows thich within two to three days. If you let alga grow freely, it will stick to a pebble and become very difficult to remove. So, sweeping work is carried out separate two days every week.