I think there are a number of reasons why there is a decline.
Yes, the seals play their part, but water quality, and depth are the main factors. Reclaiming of land has destroyed our waterways.
When the land is reclaimed, rain runs off very quickly, causing the river to quickly rise, but fall, just as quick.
With un-reclaimed land, rainfall will sit on the land, and slowly make its way into the river. The river will not rise so quickly, and then during dry spells, the land will continue to feed the river with water, and the rivers will not drop as significantly, as they do t