The Gull river at Minden has a long histor of use by the whitewater community - it is owned and managed by Whitewater ontario, and has been the site of several international slalom races. The river is quite heavily used, with weekends during the summer often booked out for races, training camps or instructor courses. The bookings can be found here; Whitewater Ontario.
The Gull does not run all of the time - it requires the dam to be releasing at least 10 cumecs. The level is usually posted regularly on the Boatwerks forum. In the average year, it does usually run for a good proportion of the summer.
The riverbed has been altered by moving rocks around to improve the features and make it usable in lower flows, firstly in 1970, and again in 2000. Despite the relatively easy grading, it is very fast, and shallow in places if you take a swim. There are several decent playwaves - depending upon levels, and Whitehorse falls halfway down. This is not so much a waterfall as a sloping ramp. There is good access to the river on both banks at the top and bottom of the run.
There is camping onsite, making for a very pleasant weekend of paddling. The campsite is also administered by Whitewater Ontario.
The river is just north of Minden, in the Haliburton highlands. To get there take highway 35 north from Lindsay, and just past Minden, take a right turn onto Horseshoe Lake road. About 4.5 km down the road, you should be able to see the river to your right. There are several parking areas, but please do not stop on the shoulder. If you reach Horseshoe Lake, you have gone a few hundred metres too far. The camping is over the bridge on Bethel road, on the right.