Disclosed fluid reservoirs include closure mechanisms for sealing an upper opening of the reservoir. The reservoir can have plates adjacent the upper opening. When the upper end of the reservoir is folded, the plates can be positioned on opposite sides, and a closure frame can be slid over the plates to pressure them together. The plates can have pins that engage with slots in the closure frame, and a separate closure lock can be slid over the end of the closure frame to prevent the pins from exiting the slots. The plates can also bias the opening open when the closure is not engaged. Also disclosed are flexible fluid reservoirs that include structural elements, such as baffles, that help retain a desired shape of the reservoir when filled or empty. Some baffles can be elastically deformable to allow flattening and expansion of the bladder while biasing toward a neutral position.