  If gravitational clustering is a hierarchical process, the present
large-scale structure of the galaxy distribution implies that structures on
smaller scales must have formed at high redshift. We simulate the formation of
small-scale structure (average cell mass: $\Delta \bar m_b=10^{4.2}$M$_\odot$)
and the evolution of photoionized gas, in the specific case of a CDM model with
a cosmological constant. The photoionized gas has a natural minimal scale of
collapse, the Jeans scale ($m_{b,J}\simeq 10^{9}$M$_\odot$). We find that low
column density ($\nhi \le 10^{14}\cm^{-2}$) lines originate in regions
resembling Zel'dovich pancakes, where gas with overdensities in the range $3 -
30$ is enclosed by two shocks but is typically re-expanding at approximately
the Hubble velocity. However, higher column density ($\nhi \ge
10^{15}\cm^{-2}$) lines stem from more overdense regions where the shocked gas
is cooling. We show that this model can probably account for the observed
number of lines, their distribution in column density and b-parameters, as well
as the cloud physical sizes as observed in gravitationally lensed quasars. We
find a redshift evolution that is
