Why were so many pre-1970 bridges built so low to the water that they do not even allow a single-level stand-up boat (serving as water buses or small fishing/research vessels) to pass underneath without raising or swinging the span? If they're built so low with short spans anyway, why not make them a pontoon bridge by making their light-duty piers floating platforms to save tons of time, money, and difficulty? Examples include the Oregon Trunk Rail Bridge (also known as the Celilo Bridge), the old Biloxi Bay Bridge (1962-2005), Norfolk Southern Lake Ponchartrain Bridge, Lake Ponchartrain Causeway, the concrete arches of the old Seven Mile Bridge (originally for railroad), etc.?
Why not build the non-shipping channel portions as high as a boardwalk pier, so that small single-floor stand-up vessels can pass under with traffic simultaneously passing overhead, just like how all bridges built since the 1970s have [been built at least as high as the Burlington Northern Railroad Bridges 5.1 and 9.6 (also known as the BNSF Railway Bridge 5.1 and BNSF Railway Bridge 9.6, respectively) across the Willamette River and Columbia River (both in Portland, Oregon and the second one also being in Vancouver, Washington), respectively]?
For example, see how the lowest spans of the Chesapeake Bay Bridge-Tunnel are built significantly higher than those of the Lake Pontchartrain Causeway, the lowest spans of the replacement Biloxi Bay Bridge (2007-present) are built way higher than those of the original (1962-2005), and the lowest spans of the replacement Seven Mile Bridge (1982-present) are built significantly higher than the original (1916-1981). Also, this phenomenon isn't limited to just the United States alone or even when also including Canada -- it is a global phenomenon. For example, look at how the lowest spans of the Annai Indira Gandhi Road Bridge (1988-present) are way higher than the those of the Pamban Bridge (1914-present) in India. The video below shows how much higher the replacement bridge (estimated completion around late 2024) for the Hanshin Namba Line over the Yodo River in Osaka, Kansai, Japan is compared to the original (1924). Also, note how the Denpo-Ohashi Bridge (the tied arch bridge, estimated to have been completed in the 1930s) in the near background is significantly lower than the Shin-Denpo-Ohashi Bridge (completed in 1958 as part of National Route 43) right behind it. All of the bridges mentioned here for this video are level. https://youtu.be/n9pBhmlQsQA