Make sure it's clean river sand, not beach sand.

确保那是干净河沙，不是海滩的沙子。

youdao

If you swim in a river or lake, be sure to investigate what is below the water surface.

如果你在河里或者湖里游泳，一定要弄清楚水面下有什么。

youdao

When I reached the Mississippi River at its source in northern Minnesota, a grocery clerk made sure to inform me that I was closer to the finish than the start.

当我到达密西西比河位于北明尼苏达州的源头的时候，当时一个食杂店的店员很肯定的告诉我，说比起出发点，现在的位置离终点更近一些。

youdao

It says it has carried out comprehensive environmental studies to make sure any impacts on the river are minimised.

BBC: Drought fears after low winter rain levels

Sure, there is the river in between, which is a barrier.

WSJ: A Diplomatic Breakthrough

"This will help to make sure people can continue to enjoy the river ready for the start of this year's spring and summer boating season, " Mr Watts added.

BBC: River Nene: Silt removal aimed at improving navigation