Hi,

We are thinking of purchasing a home -- we found out that the sewer lateral crosses a brook that is right next to the house to connect to the city main, and the sewer lateral is exposed (not buried) in the creek bed (for about 5 feet). Is this an issue? Should the pipe be buried (it's a cast iron pipe)?

only if it freezes.
remember, running water will not freeze. so if the sewer is nice and clean the water running thru it will not freeze.
but, should there be a partial restriction causing a puddle of water to lay in the bottom of the pipe, it will freeze.
or, should you have a slow dripping faucet, the water slowly trickling thru the pipe will freeze.
otherwise, exposure to the elements is no problem..

Consider that its not freezing temperatures that causes major issues, but rather those sub-freezing, bone chilling and wind chills that reach - 30, 40's and 50's that makes one wish they had a bigger heater, better protected house and pipes that were placed a foot deeper.

Are you the only in the area? Do you have neighbors with the same setup? If so, just ask them.
Best Wishes

It is immaterial. Unless you install a pump, the pipe cannot be lowered going through the wash, and if you "bury" it, the first flood will wash the dirt away. That is why it is cast iron and not some weaker material.