I am 5 minutes from 3 boat ramps on Old Hickory. It is the lake I'm on 90% of the time, year round, as I fish as well. Old Hickory is part of the Cumberland River system and does have a dam with lock. You can actually lock thru in a wake boat and make a 15 mile trip to downtown Nashville on the Cumberland River which is pretty cool. The lock also means barge traffic which hauls sand and coal from downtown Nashville up to the Gallatin Steam plant.
Is it a good lake? - I think it is a great lake, but others that are accustomed to deep, clear water highland lakes don't tend to agree.
Clean water? The water is always stained, visibility is about 2-3ft on the best of days. As far as water quality, it tests excellent, there are no alerts to contaminants in fish and its a river system lake, so the water is always getting flushed out.
Crowded?Yes, especially in the heat of summer. The best riding can be had past the hwy 109 bridge up river of the Gallatin Steam Plant in the summer.
Deep? The main river channel stays 40-70ft deep throughout, but there are tons of sandbars, mud flats and log lay downs that will destroy the bottom of a boat if you get outside of the river and creek channels. Creek channel depths probably average 15ft deep
Safe? Its about as safe of a lake as any if you stay in the channels. There is barge traffic in the main lake channel as well as big yachts/cruisers that can be hazardous to a smaller boat out in the channel. Just use common sense and invest in a good lake map your fine
Good areas for riding? Drakes Creek, Spence Creek, Station Camp Creek are all great places off the channel to find good water. Past hwy 109 bridge is the best in the heat of the summer. Don't go before May or June though because the water will be freezing up that way.