Once it becomes spring, the cherry blossoms blooming along the river are incredibly beautiful.
One of the best parts about this place is how beautiful the cherry blossoms blooming along the river are in spring.
になると = Once it becomes
春 = spring
桜 = the cherry blossoms
咲く= to bloom --> blooming
川沿いに = along the river (are)
とても = really/very --> incredibly
きれい = pretty --> beautiful
一番目の文はあなたの質問から翻訳されました。二番目のは、ノートの部分もふくんでいます。
桜は確かに "pretty" ですが、"beautiful" の方がもっと大人っぽいです。