Complex sentence: The designation of Banff as a national park marked a turning point in the public's perception of the Bow River as now not only something that had industrial and agricultural quality but an aesthetic value as well.
Split into simpler sentences: The designation of Banff as a national park marked a turning point in the public's perception of the Bow River. The river was now viewed as something that not only had had an industrial and agricultural quality but an aesthetic value as well.