Location of Lake Saawah that dried up at birth of Prophet, sallallaahu 'alayhi wa sallam
14-4-2016 | IslamWeb
Question:
Where is Lake Sawa, which once dried up due to the birth of Prophet Muhammad, sallallaahu 'alayhi wa sallam, situated?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
There is no doubt that the birth of the Prophet marked the beginning of the eradication of shirk (polytheism) and oppression on earth. However, the reports cited in the books on the biography of the Prophet indicating that Lake Saawah (Saveh) sank when he was born, are not authentically established.
Safiyy Ad-Deen Al-Mubaarakfoori 
wrote:
"...It was reported that significant precursors accompanied his birth: fourteen galleries of Kisra (Chosroes)’s palace collapsed, the Magians’ sacred fire died down, and the churches around Lake Saawah collapsed after the lake sank down. This was cited by At-Tabari, Al-Bayhaqi, and others. However, it is not authentically established with sound chains of narration and is not verified with the historical accounts of the nations in reference despite the significance of events such as these." [Ar-Raheeq Al-Makhtoom]
Lake Saawah is located in present-day Iran between Ar-Rayy (Rey) – 12 km south of Tehran - and Hamathaan (Hamazan). You can see its location on Google Maps. The geographer Yaqoot Al-Hamawi described the city of Saawah, saying:
"It is a nice city between Rey and Hamazan, and they are 22 farsakhs (about 132 km) apart. It is located near a city known as Aawah. Saawah is a Sunni city whose inhabitants followed the Shaafiʻi School of Fiqh, and Aawah is a Shiite city whose inhabitants follow the Imaami Shiite school, with 2 farsakhs (about 12 km) between the two cities. Conflicts continued to arise between them until 617 AH, when the Turkic Tatars invaded Saawah and killed all its people. It had a library bigger than any other in the world, and I was told that they burned it down ... The hadeeth reported on the authority of Sateeh, the monk, indicated that the fire of the temple of Persia (which they worshiped) was extinguished and that the lake of Saawah dried out on the night of the birth of Prophet Muhammad as cited in Aʻlaam An-Nubuwwah..." [Muʻjam Al-Buldaan]
Ar-Rawdh Al-Miʻtaar fi Khabar Al-Aqtaar by Abu ʻAbdullaah Al-Himyari (d. 900 AH) reads, "Saawah is a village between Rey and Hamazan in Iran, and they are 22 farsakhs apart. The report narrated on the authority of Sateeh, the monk, about the interpretation of the vision seen by Khosrow Parviz, the King of Iran (Persia), included that he saw that his palace was rocked, fourteen of its galleries crashed, the fire of the temple of Persia was extinguished, and the water of Lake Saawah dried up..."
These citations indicate that the Lake Saawah of which it was said that it dried up when the Prophet was born is the one located in Saawah city in Iran, between Rey and Hamazan, both within the Iranian borders.
It should be noted that there is a lake called Saawah as well in Iraq, to the west of Samawah in southern Iraq.
Allaah knows best.