In the U.S., hydropower is expected to play an important role in supporting a zero-carbon energy transition so it's becoming increasingly important the hydropower regulatory process provides robust environmental and other protections while maintaining regulatory efficiency. In this study, we created a dataset of project and license characteristics and milestones using hydropower licensing documents from 107 randomly selected projects to 1) quantify the length of steps in the licensing timeline and 2) quantitatively identify factors associated with licensing timeline length and sources of uncertainty. We found original licenses had shorter average timelines than relicenses and project capacity was only related to timeline length when license type (i.e., original, relicense) was included in analyses. Both licensing timeline length and variability were impacted by the licensing process (Alternative, Integrated, Traditional) used. Projects with greater environmental complexity (e.g., endangered species) had significantly longer timelines than projects that were less environmentally complex. We attributed shorter timelines for original licenses to lower environmental complexity (e.g., no endangered species) because most original licenses involved development of already impacted sites. Projects with greater environmental complexity significantly impact resources that may require greater stakeholder involvement and increased study that can lead to longer licensing timelines.