One of my favorite elementary science sites is sciencenewsforkids.org
Applications for Science Teachers: Science News for Kids is a great source of short expository text to use for teaching/supplementing instruction in a science concept and also teaching a reading comprehension strategy. Quickly print off a short piece of text that students can write on to practice a reading strategy. The Teacher Zone also features an article with supporting questions each week. The site could be a good source of information for student research.
You may want to check out Science News.