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Is there any difference in meaning between those two sentences concerning the use of (along) vs. (by)?
1- Cities must have water, and so they are often built along rivers.
2- Cities are built in safe, suitable places, such as by rivers.
Can they be equally replaced?
1- Cities must have water, and so they are often built by rivers.
2- Cities are built in safe, suitable places, such as along rivers.
Yes in this case along and by are interchangeable
Along or alongside can be used when the meaning is next to.
by can be used as a preposition, adverb meaning near or beside(in distance)
If you need to reference all are available in the Cambridge English Dictionary online Link to C.E.D.
Yes, there can be a difference, but note that they can be used synonymously.
According to the Merriam-Webster ESL Learners Dictionary, which is found here:
along
1 : in a line matching the length or direction of (something)
We walked along the beach.
We walked along (the side of) the road.
The chairs were lined up along the wall.
The ship sailed along the coast.2 : at a point on (something)
They have a house along [=alongside] the river.
We drove to Boston and we stopped along the way for lunch.by
1 : close to or next to (something or someone) : near
She was standing by [=beside] the window.
His wife was sitting by him.
They have a house by the lake.
So, along rivers can mean matching the length of rivers, and by can mean near. Your example sentences cannot be equally replaced.
If we want to thoroughly split hairs, I suppose we could come up with a slight difference.
Any city that is built along a river can also said to be by a river.
However, not every city that is built by a river can be accurately described as being along a river.
Consider this map:
Both cities are by the river (we could also say they are on the river), but one could argue that the northern city is built along the river, while the southern city is built more near the river than along it.
However, most of time time, people won't think that much about this subtle difference. And nobody would argue that the northern city is not by the river, because clearly it is.
Also, in a practical sense, most cities probably take up enough riverfront land that it would be very hard to find an example of a city that would be considered by a river but not along the river.
