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 The Aa is a tributary of the Kleine Nete in the basin of the Scheldt, in the Belgian province of Antwerp. It's formed by the drainage of the area north of Turnhout.
In the old days there used to be iron ore in the Aa valley. This had to be brought to unloading places on the Kleine Nete. To realize this, several small channels were dug between the Aa and the Nete.
Beyond the path from Tielen to Gierle, the river is powerful enough to power the ancient water mill of Tielen. This is located on a little island between two branches that are called the Old and the New Aa.
 From the 1970's, the valley was "poldered" for agricultural purposes: the river was straightened and embanked. Locks were built for the regulation of the water level. In recent years these locks have gotten a by-pass so the fish can travel freely up and down the stream.
The Aa flows into the Kleine Nete near another historic watermill, at Grobbendonk. This mill is one of two watermills who are still in use commercially in Flanders to this day. 
The valleys of Aa and Kleine Nete together form a continuous green area, with the Schupleer nature reserve at its heart. (source: trobken kajako)

Kayaking on the Aa is not a very challenging experience because the current is quite mellow. Only after heavy rains it gets a bit more demanding. During every season it is possible to start downstream against the current, or upstream and turn back after a while.
Because there is no commercial kayaking on the Aa there are no landing places or jetty's anywhere. Due to recent changes at the junction of the Kleine Nete and the Aa it isn't so easy anymore to launch your kayak on the Aa.
But is a nice river to paddle, if only to see the sometimes disastrous changes our ancestors have made to a perfectly fine river in the name of commerce.
It is a good thing that in recent years there have been a lot of projects trying to repair this historic damage.