  We study the Poisson geometry of the first congruence subgroup
$G_1[[z^{-1}]]$ of the loop group $G[[z^{-1}]]$ endowed with the rational
r-matrix Poisson structure for $G=GL_m$ and $SL_m$. We classify all the
symplectic leaves on a certain ind-subvariety of $G_1[[z^{-1}]]$ in terms of
Smith Normal Forms. This classification extends known descriptions of
symplectic leaves on the (thin) affine Grassmannian and the space of
$SL_m$-monopoles. We show that a generic leaf is covered by open charts with
Poisson transition functions, the charts being birationally isomorphic to
products of coadjoint $GL_m$ orbits. Finally, we discuss our results in terms
of (thick) affine Grassmannians and Zastava spaces.
