I do not own a PowerShot G7 X, but my friend does.
We want to know if it is possible to adjust the aperture when in [Tv] mode, so that she can try for milky water on a waterfall.
Is this possible?
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The whole point of Tv mode is that you don't adjust the aperture; the camera meters the scene and calculates the aperture based on the selected shutter speed and ISO setting. If you want to adjust the aperture, use Av or M mode.
As to the "milky water", I think what you're actually looking for is a slow shutter speed to blur the water. Your aperture setting isn't as important here. In Tv mode, set a slow shutter speed and use a tripod to avoid camera shake. Since a long shutter speed is required to capture this effect, conditions can simply be too bright for the shutter speed needed, so use a low ISO setting, and possibly also a ND filter.