“Statues of Memnon at Thebes, during The Inundation”, a lithography from 1848 by Scottish artist David Roberts. It appeared in Roberts book on the Nile region entitled “Egypt & Nubia” published in London the same year.
The flooding of the Nile has been an important natural cycle in Nubia and Egypt since ancient times. It is celebrated by Egyptians as an annual holiday for two weeks starting August 15, known as Wafaa El-Nil. It is also celebrated in the Coptic Church by ceremonially throwing a martyr’s relic into the river, hence the name, The Martyr’s Finger (Coptic: ⲡⲓⲧⲏⲃ ⲛⲙⲁⲣⲧⲏⲣⲟⲥ, Arabic: Esba` al-shahīd). The flooding of the Nile was poetically described in myth as Isis‘s tears of sorrow for Osiris when killed by his brother Set. (source wikipedia)
For a similar rite see my post on the Venetian holiday “Lo Sposalizio del Mare” (the Marriage to the Sea) here.