This site is new news to me, and great for some layout and design situations: https://nhd.usgs.gov/
Although it’s not practical or possible to always bypass the lower 1/8-1/4 of any mountain to lower the chances of hitting seeps as a result of “toe-of-slope-effect,” the site (with the help of GIS) could be used to lower the potential of running into issues with slowing down surface and subsurface water as it goes from upslope to trail to downslope. It can also help in confirming suspicions and concerns about slope stability and water holding capacity on slopes steeper than the angle of repose for the soil and/or bearing capacity and shear stresses in question.
At a future date I will elaborate on the words above, but for now this page, like many on this site, is a reminder for me that I have one more tool (or link) for trail building. I extend my thanks to Mike Shields for helping me see topography in a whole new way, and recognizing how I can use that information for trail construction.