1To move from downstream to upstream along a river or current — used physically for fish swimming upstream to spawn, boats heading toward a river's source, etc. The kanji 遡 became jōyō in 2010, so 遡る is the more common written form for this literal sense, though kana さかのぼる appears in softer prose.
鮭は産卵のために川を遡って生まれた場所まで戻る。
Salmon swim back upstream to their birthplace to spawn.
舟で川を遡り、奥の集落まで物資を運んだ。
They went upstream by boat to deliver supplies to the inland village.
この川を上流に遡っていくと、美しい滝にたどり着く。
If you go upstream along this river, you'll reach a beautiful waterfall.
2To trace back through time, events, or causes — heading back toward the origin. Common in 歴史をさかのぼる (trace back through history), 記憶をさかのぼる (cast one's mind back), and 原因をさかのぼる (trace the cause). Also used in legal/institutional contexts to mean 'retroactive,' e.g., 〜年前にさかのぼって適用する (to apply retroactively from X years ago). Both kanji 遡る and kana さかのぼる are common in this sense.
この神社の歴史は平安時代までさかのぼる。
The history of this shrine dates back to the Heian period.
問題の原因をさかのぼると、最初の設計段階に行き当たった。
When we traced the cause of the problem back, we ended up at the initial design stage.
値上げは四月一日にさかのぼって適用される。
The price increase will be applied retroactively from April 1st.
彼女は遠い昔の記憶を遡って、子ども時代の出来事を語り始めた。
She cast her mind back to distant memories and began recounting events from her childhood.
家系をさかのぼってみたら、意外な共通の祖先が見つかった。
When we traced our family line back, we found a surprising common ancestor.