Why did they call the Han River a lake?

Even in winter, the Han River doesn't reveal its bottom these days. But 40 years ago, during the dry season, sandbars would prominently appear, and during the rainy season, it would flood over the banks. It all started in 1982 when they dredged a 36km stretch from Amsa-dong to Haengju Bridge, fixing the waterway to a depth of 2.5m. However, digging deeper caused the water to drain even faster, and saltwater flowed back upstream during high tide. The solution wasn't to dam the estuary, but to build underwater steps. A submerged low weir beneath Jamsil Bridge is what currently holds the Han River's water level.