the phrase "line with"
The Los Angeles River was lined with concrete in the late 1930s and 1940s, after years of periodic flooding.
what is "lined with concrete"?

Question

Answer

It means they put concrete underneath and around the sides of the river. (Like a pipe with no top) If you do a google image search of the LA river, you'll see what I mean. It's used in many things though to covey an under/inner layer. Boots lined with fur to keep you warm (fur on the inside) A lace dress lined with satin (satin would be the layer underneath the lace)

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