A natural lake that is fed by river water and springs can be used for immersion. Please see Natural Spring as Mikveh for further information about the use of natural bodies of water and concerns that must be taken into account (e.g., safety, modesty, and adequate lighting to assure that immersion is complete).
As long as the immersion is before alot hashachar halachic dawn), it is still considered immersing at night. Immersion between alot hashachar and& sunrise is permissible in a case of need.
Please feel free to get back to us with any further questions.