One of the requirements for a mikveh is that a body of water may not be artificially redirected or rechanneled. Does that mean, for example, that an entire river downstream of a dam, either for preventing floods, generating hydroelectric power, or asserting water rights, is invalid for use as a mikveh?
From a Rabbi Jachter article here:
[T]he Halacha provides for a number of ways to remedy the Mayim Sheuvim status of water that had been in a receptacle. One way is the process of Hamshacha, running the Mayim Sheuvim along the ground. The Gemara (Temura 12a) cites a Braita that teaches that if the Mikva has a base of more than twenty Saah of rainwater that went directly into the Mikva, then one may obtain the rest of the forty Saah by taking Mayim Sheuvim and running it along the ground on its way into the Mikva. The Rambam (Hilchot Mikvaot 4:8) and Shulchan Aruch (Y.D. 201:44) codify this Braita as normative Halacha. For a conceptual analysis of the process of Hamshacha, see Rav Yitzchak Zeev Soloveitchik in his commentary to Temurah 12b. We should note that the Radbaz (Teshuvot 1:85) writes that this process is acceptable only Bdiaved (post facto) and should not be relied upon Lechatchilah (initially). Rav Moshe Feinstein (Teshuvot Igrot Moshe Y.D.3:64:3) and Rav Yonatan Shteif (Teshuvot Mahari Shteif 142) rule in accordance with the Radbaz.
There are a number of major disputes that pertain to the process of Hamshacha. One debate is the status of a Mikva that is created entirely through the process of Hamshacha. The Rambam (Hilchot Mikvaot 4:9) cites an anonymous “few sages from the West” who believe that a Mikva is acceptable even on a Rabbinic al level even if the entire Mikva was created through the process of Hamshachah. Some argue that the Rif and Rashi subscribe to this view. The Rambam strongly rejects this view. The Chazon Ish (Y.D.130:14) writes that he believes that the Raavad believes that a Mikva is disqualified on a Torah level if all of its water entered via Hamshacha. Tosafot (ibid.) suggest an intermediate position that a Mikva whose water entirely consists of Mayim Sheuvim revitalized through Hamshacha is only disqualified on a Rabbinic level. Many (but not all) later authorities adopted this approach. These authorities include Teshuvot Tashbeitz (3:12), Teshuvot Maharit (Y.D.2:17), Teshuvot Divrei Chaim (introduction to Hilchot Mikvaot, number 5), Yeshuot Yaakov (201:15), and Chazon Ish (Y.D. 126:1 and 130:14).
Thus, it would appear that Hamshacha would not apply in a situation where the river water became She'uvim through being dammed.