Thank you all for valuable replies. For variation I might go both with trestles on masonry foundation and my other suggestions. The important thing for me is to give it a rugged look that convinces.
KIM it could change in time, a railroad being built in the 1860's-70's would often hastily put in timber trestles to get trains running, then later in the 80's-90's go back and replace them with larger more permanent bridges of iron or stone. So in your period you might a wood bridge and an iron one along the same line at different places.
They had iron bridges in those days as well. In the early years they just took a form that they were familiar with building in wood, like the box truss, and reproduced it in iron. For example:
I have figured out what is wrong with my brain! On the left side nothing works right, and on the right side there is nothing left!
After buying one of these kits, you may want to "copy" the kit for second or third bridges. Both of these are piece by piece kits and can be shortened or lengthened by adding/subtracting materials. My
either would be a good choice for 1870-1890 Colorado although trestles were generally the preferred form. In crossing white water a masonary cutwater footing would be built in the stream, the top of which would be above any any conceivable high water. The cutwater would face upstream to break up any ice which might cause a jam and flooding in the river.
I currently in the research phase for my layout which will be freelanced and set somewhere in the rockies. Of course I will have wooden trestles to span ravines as focal points, but when it comes to bridges across rivers I am a bit uncertain. I guess a wooden trestle with its bents down in a raging river would not be realistic in the real world...
So here is a few candidates that I consider for the purpose and era:
http://www.internettrains.com/merchant2/merchant.mvc?Screen=PROD&Product_Code=CSM-762
http://www.internettrains.com/merchant2/merchant.mvc?Screen=PROD&Product_Code=MID-3053
Any other suggestions? Views?