"...the brook of the willows..."

Reading Isaiah is a study in serenity for me.  I don't understand half of it, and I just breathe in and breathe out so I can enjoy the poetic aspects.

Isaiah 15:7

7 Therefore the abundance they have gotten, and that which they have laid up, shall they carry away to the brook of the willows.

This chapter looks to be a premonition about evil and punishment, so that is how I approach this.  You had an abundance, you put in the effort to get it, and "they" who ever is dominating the situation is taking your stuff to "the brook of the willows", which sounds very picturesque, but is likely some ritual sacrifice place.

Your abundance (perhaps righteousness) is up in flames under the boot of some foreign power that can make you surrender things.  There are places where you must abandon your own devotion and follow the conqueror's religion. 

Moses already taught the people what would happen: 

Deuteronomy 28:64

64 And the Lord shall scatter thee among all people, from the one end of the earth even unto the other; and there thou shalt serve other gods, which neither thou nor thy fathers have known, even wood and stone.

This comes from disobedience to the Lord. You don't honor him and the liberty he provides, you will be forced to serve a false god as a slave.   God will hang you out to dry for a while and help you see that obeying the Lord makes you free.