A mature river is characterized by a slow flowing current with lots of meanders flowing across a relatively flat plain where the grinding action of the stream load will tend to grind any Gold present into fine particles like flour that are very difficult to recover. Because of this many of the larger streams are overlooked as places where you can find placer gold. However, there are a few places in one of these rivers that might be a profitable place to look.
For the purpose of this discussion we are going to use the upper reaches of the
Connecticut River between and Vermont as an example, but the same situation occurs in all large rivers that are located in gold bearing areas. New Hampshire
The first situation occurs at the mouth of a gold bearing stream where it enters the larger river. Since for the most part gold is found along the banks of streams and the edge of water because of its great density it does not like to migrate downstream except under the influence of some fast flowing water that is usually represented by a flash flood or spring freshet. In the case of its entering a larger river there is a sudden drop in the velocity of the current at the mouth of the stream that will cause gold to be deposited at the mouth of the stream and along the water's edge in the larger river.
The other situation where gold was found is that the larger river encounters a waterfall where the gold in the river is set to be deposited in either the cracks and crevices of the rock that forms the waterfall, or in the plunge pool beneath the falls.
It should be remembered that most gold is transported along a stream by the action of episodes of flash flooding rather than the slower moving currents that are encountered during the summer. This is true of all rivers or streams.
The final place where it might be profitable to search for Gold in a large river is where a floating tree or other debris forms a natural riffle in the bed of the river.
Although you can test for the content of gold with a simple gold pan, but for more serious mining you should use some variation of a sluice box for maximum gold recovery. One of the more effective of these devices is the recently invented GoldCube.