A common way for students to develop scientific argumentation abilities is through argumentation about socioscientific issues, defined as scientific problems with social, ethical, and moral aspects. Computer-based scaffolding can support students in this process. In this mixed method study, we examined the use and impact of computer based scaffolding to support middle school students' creation of evidence-based arguments during a 3-week problem-based learning unit focused on the water quality of a local river. We found a significant and substantial impact on the argument evaluation ability of lower-achieving students, and preliminary evidence of an impact on argument evaluation ability among low-SES students. We also found that students used the various available support--computer-based scaffolding, teacher scaffolding, and groupmate support--in different ways to counter differing challenges. We then formulated changes to the scaffolds on the basis of research results.