Following Mincha services of the first day of Rosh Hashanah, it is customary to go to a seashore, river, well, or pit in order to recite the order of “Tashlich.” If there is no river, lake, or pond in close proximity of one’s vicinity, it is likewise perfectly acceptable to recite the order of Tashlich upon a bucket of water or a running hose or faucet. This was indeed the custom of Maran Rabbeinu Ovadia Yosef zt”l who would recite “Tashlich,” along with his entire congregation, on a bucket of water. This custom, along with the reason behind it, is mentioned in the book of customs authored by Rabbeinu Yaakov Molin: “It is customary to go to a sea or river on Rosh Hashanah in order to (symbolically) throw all of our sins to the depths of the sea.” The reason for this custom is based on the Midrash which states, “When Avraham Avinu went with Yitzchak to Mount Moriah to offer him as a sacrifice as he was commanded, the Satan preceded them on the way and began to persuade Avraham against this and prevent him from fulfilling Hashem’s commandment. When the Satan saw that he was unsuccessful in persuading Avraham against sacrificing Yitzchak, the Satan went before them and turned into a large river so that they would not be able to cross. Nevertheless, Avraham and Yitzchak continued on their journey unperturbed and entered the water until it reached their necks. Avraham Avinu then lifted his eyes heavenwards and exclaimed: ‘Master of the World! You have appeared to me and told me, Take Yitzchak, your only son whom you love, go to the land of the Moriah, and bring him there as an offering. I have not thought twice about this and I have not hindered your command. If we drown in this river, who will sanctify your great name? Save me G-d, for the water have reached my very soul!’ Immediately, Hashem castigated the Satan and he disappeared.” The holy Zohar states that Akedat Yitzchak (the binding and near-offering of Yitzchak) occurred on Rosh Hashanah; it is for this reason that the Torah portion read on Rosh Hashanah deals with Akedat Yitzchak. Similarly, it is for this reason that we customarily go to river or pond on Rosh Hashanah in order to recite Tashlich on Rosh Hashanah in order to commemorate the merits of Avraham and Yitzchak.
Another reason for the custom of Tashlich is recorded in the Siddur Aholei Yaakov which is that the Gemara (Horayot 12a) states that when a new king is anointed, he is anointed next to a spring to symbolize that his reign should be continuous. Similarly, Rav Mesharshia told his children that when they study Talmud, they should do so at the seashore or at the river bank to symbolize that just as these waters flow continuously, so should your Torah study flow continuously and you should never forget it. Since we perform many symbolic actions on Rosh Hashanah as a good sign for the coming year, it is therefore customary to go to the river to symbolize that Hashem’s mercy and kindness should flow continuously to us and that He inscribe us in the Book of Life for a good and peaceful life and that He forgive us for all of our sins, as the verse states, “I am hereby turning to you like a river of peace.”