Through theory and numerical simulations in an axisymmetric geometry, we examine evolution of a symmetric intrusion released from a cylindrical lock in stratified fluid as it depends upon the ambient interface thickness,
$h$
, and the lock aspect ratio
${R}_{c} / H$
, in which
${R}_{c} $
is the lock radius and
$H$
is the ambient depth. Whereas self-similarity and shallow-water theory predicts that intrusions, once established, should decelerate shortly after release from the lock, we find that the intrusions rapidly accelerate and then enter a constant-speed regime that extend between
$2{R}_{c} $
and
$5{R}_{c} $
from the gate, depending upon the relative interface thickness
${\delta }_{h} \equiv h/ H$
. This result is consistent with previously performed laboratory experiments. Scaling arguments predict that the distance,
${R}_{a} $
, over which the lock fluid first accelerates increases linearly with
${R}_{c} $
if
${R}_{c} / H\ll 1$
and
${R}_{a} / H$
approaches a constant for high aspect ratios. Likewise in the constant-speed regime, the speed relative to the rectilinear speed,
$U/ {U}_{\infty } $
, increases linearly with
${R}_{c} / H$
if the aspect ratio is small and is of order unity if
${R}_{c} / H\gg 1$
. Beyond the constant-speed regime, the intrusion front decelerates rapidly, with power-law exponent as large as
$0. 7$
if the relative ambient interface thickness,
${\delta }_{h} \lesssim 0. 2$
. For intrusions in uniformly stratified fluid (
${\delta }_{h} = 1$
), the power-law exponent is close to
$0. 2$
. Except in special cases, the exponents differ significantly from the
$1/ 2$
power predicted from self-similarity and the
$1/ 3$
power predicted for intrusions from partial-depth lock releases.