Final answer:
The observer saw a heron wading in the Rhine River, as the anagram 'an observer see a heron wading' contains the scrambled name of the river 'heron', which corresponds to 'Rhine'.
Step-by-step explanation:
The anagrammed description of a European river where an observer might see a heron wading hints at the name of the river itself being scrambled. Given the options of the Danube River, the Rhine River, the Seine River, and the Thames River, the clue can be deciphered. When unscrambling the phrase 'an observer see a heron wading,' the word 'heron' aligns with the 'Rhine River', suggesting that this is the intended answer. Therefore, the observer saw a heron wading in the Rhine River.