What is a Broomie Law?

The online Dictionary of the Scots Language (http://www.dsl.ac.uk/) defines 'law' as:

Law, n.² Add: b. `A mound of earth and shingle on the bank of a river on to which salmon nets are drawin to be emptied'

It may be deduced that the section of the north bank of the Clyde west of the city centre was a place where whin or broom thrived, hence the Broomielaw.