A. Because they rub against the banks, and are delayed in their current thereby.
Q. Why does soapy water bubble?
A. Because the soap makes the water tenacious, and prevents the bubbles from bursting as soon as they are formed.
Q. Why will not water bubble without soap?
A. Because it is not tenacious enough to hold together the bubbles that are formed.
Q. When soap bubbles are blown from a pipe, why do they ascend?
A. Because they are filled with warm breath, which is lighter than air.