  Hack [Studies of longitudinal stream profiles in Virginia and Maryland
(1957). Report], while studying the drainage system in the Shenandoah valley
and the adjacent mountains of Virginia, observed a power law relation $l\sim
a^{0.6}$ between the length $l$ of a stream from its source to a divide and the
area $a$ of the basin that collects the precipitation contributing to the
stream as tributaries. We study the tributary structure of Howard's drainage
network model of headward growth and branching studied by Gangopadhyay, Roy and
Sarkar [Ann. Appl. Probab. 14 (2004) 1242-1266]. We show that the exponent of
Hack's law is $2/3$ for Howard's model. Our study is based on a scaling of the
process whereby the limit of the watershed area of a stream is area of a
Brownian excursion process. To obtain this, we define a dual of the model and
show that under diffusive scaling, both the original network and its dual
converge jointly to the standard Brownian web and its dual.
