they picked up trash that washed down the rivers during recent floods. 

In central Japan, we’re having hotter summers, colder winters and more
unpredictable and severe weather in between. This spring sudden
downpours damaged crops, and washed garbage down river (which we did our
best to pick up). This particular “dot” we labeled. It was nothing to
the freak tornado that devastated part of Tsukuba, the hail storms
(marbles and golf balls), and the killer lightening storms that plowed through three prefectures just afterwards,