We prefer to keep it simple when it comes to whistle blasts, which are primarily used to get the attention of the rest of your team so that you can provide additional information through the use of hand or paddle signals.
In personal experience one and three blasts are most commonly used. One is often used to get the attention of inattentive paddlers or the attention of leading paddlers by those in the back of the group. Three are used to alert the group that something has gone wrong. A good paddling group will often touch base here and there verbally (by leapfrogging in eddies, etc) that requests to stop can typically be handled in this way. Nevertheless, if you’re on the river and hear two whistle blasts, you should pull off and seek further information.
Typically, whistle blasts are followed by more specific information in the form of hand signals, to which we now turn.