Basically plants have to be divided every few years. That means pulling the entire plant out of the pond and cutting the root ball in 1/2 or 1/3 or 1/4. Re pot or re plant one chunk and throw out the rest.

The root ball can be cut using a shovel, or knife or teasing the roots apart. When plants have be let go this long generally a shovel is needed.

Because of the size of the pond, number of plants and size of the plants I think it would be best to drain the pond, divide the plants and then refill. Normally I'd never do this, but that is more of a water garden than fish pond and being a commerical property you don't want to be messing around.

If the plants are in pots the pots are probably split and you'll need new pots. Also I buy fruit tree fertilizer spikes, broken into chunks, place a few of them into the pot. Cheap, most places that sell plants have them.

This is a big job, like maybe 2 days for a first timer. And the pond would probably not look good for a few weeks. I'd let mgmt know in advance so they can agree and not be unhappy when they see the mess.

Another, easier choice it to just remove some plants completely and throw them out. Keep doing the that until it looks the way you want. You can place an ad on CraigsList and you might get someone to do the job for you for free. But dealing with CL people is a chore too.

The plants around the pond look like they could use dividing too. That could help the pond look less overgrown. Kind of want to balance everything.

Basically the iris can be thinned by pulling out sections. You may not have to dig up all of it, I never do. I don't dig up my waterlilies either I just reach in and tear off sections of rhizome.

I really don't think it looks bad. It looks like a jungle and that is a good thing. The waterlilies are humped up though because they do need to be repotted and divided.

It looks to me like most stuff isn't in the pond/stream anyway. But to the extent that it is, you just pull stuff out in clumps and work with the shovel when the clump is outside of the liner so that you don't damage the liner. You may find that the plants have stuck aggressively to the liner and it will be disturbed as you drag/yank large clumps out. You could get waders and stand in the stream to prevent the liner from moving.

thanks for the replies. There are fish in that pond. The water didn't run for a month and it got really gross in there. All the plants seemed to be wedged into cracks no pots are visible.

rip those iris out and divide them. that will open the pond right up

The only thing that I would do with the pools is to get out the hedge shears and just cut back the tops of all the tall growing plants to where you can see the water. I would not disturb the roots at all or do any digging at all. The plants are healthy and there appears to be plenty of water that is not being filled in with plant material.
If you can see plants growing out into the water and see
the rhizomes then you can trim off just those extensions but otherwise leave the plants alone. This will make a minimal impact upon the scene and yet keep the plants more or less intact.