Leak in stream? (long)

Hello all,
I'm posting today to ask for help and tips in locating a leak in what I now
believe is in my stream. Here's the history,
I week ago I added some small river pebbles to my stream because I thought the
bigger river rocks needed some fill to look right.Then on wednesday I noticed
that the pond level had dropped about 3-4 inches in just a couple days. (not
good) BTW that's the only change that i've made to the pond/stream filter/etc.
in a couple months and all has been well since then for this Rec Pond newbee.
Yesterday morning I filled the pond again only to check it 12 hours later and
saw a drop of about and inch and a half. anyway last night I turned off the
pump for the filter/falls and figured I'd be able to tell in the morning
weather the leak was in the pond , filter or the hoses leading to the filter.
This morning (12 hours later) when I went out to check on the situation I
noticed that the pond level was as left and the filter level was also the same.
The filter lines have a check valve at the beginning of there run so the filter
water pressure was pushing on them all night yet the filter level was still
full. However the stream level was almost empty and it should hold a couple
inches when the filter is off. I haven't moved any rocks or any of the stream
liner recently so I don't think that the water would all of a sudden be
spilling over the liner. But, I do have a dog and I'm thinking that maybe he
has stepped into the stream and his nails might have torn the stream liner. So
later on today I'm planning on removing alot of the river rock from the stream
in search of a punchure in the liner. I'm posting here to ask if you think that
I'm on the right track here or do you think that I may have overlooked
something? PS it's been raining here in Philly so all the ground is wet so I
can't use that telltale to help me.
Thanks to all for any thought and/or tips you may be able to provide,
-Pat
(freaking with the leaking)