There is a formula that is used to calculate the amount of flow of a river at a bar. It is called the "rule of 12". This flow will be best described as:
For each hour after the tide change the flow will be:

1st hour will be 1/12th
2nd hour will be 2/12ths
3rd hour will be 3/12ths
4th hour will be 3/12ths
5th hour will be 2/12ths
6th hour will be 1/12th

From this table you can see that the maximum flow will be the middle 2 hours of an exchange. This equates to the bar being roughest at that time. Wind conditions on any tide, will extend these times. All else taken into consideration, the bar usually tends to not be as rough on the incoming tide.