This is the 3172nd most frequent Russian word.
множеством
"Множеством" means "a multitude" or "a set" in English.
The word 'множеством' in this context is used in its instrumental form, denoting a large quantity or multitude.
Вокруг находилось множество цветов.
Around there were many flowers.
The word 'множеством' in this context illustrates the cause or the entity causing the emotion, emphasizing the surprising multitude or abundance.
Он был удивлен множеством вопросов.
He was surprised by the multitude of questions.
Here, 'множеством' indicates the surrounding amount or multitude of trees in proximity to the river.