Let's go!

    My knowledge of music is almost nonexistent but, the way I understand the definition, a riff is a short, simple melody which can be used as part of a larger composition but which has its own identity and can be played and recognized on its own.  For instance, the well-known Chinese / Asian riff.  I suppose other examples of riffs would be the first few notes of some well-known songs like Jingle Bells or Happy Birthday.
    What I don't know is if an even simpler composition, with only one note and some rhythm would qualify as a riff.  I guess we could call it a "one note riff".  One example I have in mind is the "Let's Go!" used by cheerleaders and fans at sports events worldwide -- you can hear it here-- and which can be heard from the horns of cars in Spanish cities after the local team has won a match.  Wikipedia says it was created by a musical group called The Routers (!) in 1962 although it is so simple I suspect it may have existed earlier and they just used it in their song. 
    I had a look at the rhythm and found out it is very simple.  Clap or tap on the table fifteen times in a row spaced 1/4 seconds and you get this (download to play). 
    Listen to it one or two times and you can reproduce it without counting the taps.  Our minds are very good at looking for patterns (and rhythm is one kind of pattern) and, even though it is just 15 taps all spaced evenly, it mentally grouped them into three groups of four taps and one last group of three taps.  You can easily reproduce the sequence without need to count the taps as you go along.  Just follow the "rhythm".
    Now, keeping the timing, remove the second, fourth, eighth and thirteenth taps and this is what you have.  The Let's Go one note riff.  The note separations are 2,2,1,1,2,1,1,1,2,1. 

    As the brain tries to find melody in the rhythm it may seem like all the notes are not exactly the same pitch and some are higher than others and sometimes one note riffs are played by musicians using different notes.
Shave and a haircut

    This is another well-known one note riff.  In case you do not know what I'm referring to you can hear it here.  This is originally a single note riff as it is tapped or knocked on any surface but musicians will play the first and two last notes higher.  This page gives the notation illustrated at right. 
    I easily made the WAV file using the following parameters: 7 taps separated 2,1,1,2,4,2 units, basic separation unit 150 ms.