The importance of high-tech careers has risen as the international competitiveness of US students on tests of science and math has declined. The problem is compounded by a persistent inequity in STEM participation and achievement in the US across gender and racial and ethnic groups. Bridgewater State University's Student Retention Enhancement Across Mathematics and Science (STREAMS) program has instituted simultaneous, structural changes across every introductory science and mathematics course. These changes include a required peer-led cooperative-learning program that attaches learning supports to an interconnected series of gateway science and math courses in order to improve retention in the STEM majors. This paper begins by describing the characteristics of students supported by STREAMS. It then describes how STREAMS personnel worked with departments to design and implement structural changes to their introductory courses, along with the key features of the models adopted by each department and how they fit into a range of peer-led cooperative-learning models. It shows the results of these changes on student retention within STEM majors for all students and for traditionally underserved students.