Heavy rain across Taiwan last week brought much-needed water to the island’s reservoirs, especially in the south, which had been in a drought. But in Taiwan, heavy rain brings with it an unwanted side effect: silt that clogs up reservoirs and requires clean-up operations.

The annual wet “plum rain” season that marks the beginning of summer in Taiwan has arrived. This year, its torrential downpours have been a particular relief to Taiwan’s parched south. But while reservoirs are swollen, it isn’t just with rainwater.

Silt and mud get carried with the rain by streams rushing down from the mountains. All this muck reduces the capacity of the reservoirs these streams help fill.

In Nantou County’s Wushe Reservoir silt deposits have gotten high enough to create sand bars. The head of a hydroelectric plant that uses this reservoir’s dam says that to solve this problem, engineers are going to have to dig a sizable tunnel for channeling silt away.

The Water Resources Agency says it is working on building such tunnels in addition to keeping up silt pumping operations at Taiwan’s reservoirs.