结构分析：

这句话的主干是前面部分，as heavy rain continues to是as引导的时间状语从句。swelling rivers and reservoirs是现在分词作结果状语。

3.Engineers are pumping water through the sluice gates to try to control the water levels, but they have to be careful not to allow too much to flow downstream where communities are already waterlogged.

参考翻译：

工程人员正在试图通过水闸泄水来控制水位。但是他们必须非常谨慎，不能让过多的水涌入下游，那里的社区已经被水浸。

结构分析：

这句话的主干是：Engineers are pumping water through the sluice gates。to try to control表示目的。where communities are already waterlogged是定语从句，修饰downstream。