Outside of simply agrarian production, what type of industry could a lord in the Riverlands start up?

Well…a lot of extractive-based industries are excluded by geography – not much in the way of mining, thus not much metalworking; not much in the way of timber, thus can’t really get into woodcrafts and the like. 

On the other hand, the Riverlands potentially have a good bit of water power. So textiles might be a good bet. Flax grows well near water or in watery soil, so linen production might be a good way to go.