I have to admit that I really know fairly little about the field of Hydrology. Basically I just found what I thought was a pretty cool cache location and tried to figure out a subject that it would fit with for the NoVAGO U. series. Here is the Wikipedia description and that seems to fit pretty well with this cache:
Hydrology (from Ancient Greek ὕδωρ (húdōr) 'water', and -λογία (-logía) 'study of') is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability.
I found this particular location while doing a little hiking with RustyCoin one day. Then I came back several weeks later and I was pretty sure I knew where the spot was but I couldn't find it. Now granted it was supposed to be surrounded by water but I eventually figured out that it was UNDER the water. And it definitely hadn't been the first time I was there. It had just rained quite a bit the past day or so and I figured that the water must have risen because of the rain. I do have a little knowledge about rain and rivers and I know it usally takes a while for all the creeks to flow into the rivers so sometimes it takes a while for a river to crest after a rain. The weird thing here was that after being there for 20 minutes or so the spot where I wanted to put my cache started to slowly appear. But it had only just recently stopped raining and I really thought the river should be rising or at least not dropping its level so quickly. But sure enough it dropped several inches in the time that I was there. So I went back home and did some research and found out that the tide extends that far up the Potomac. I guess it actually extends a couple miles further upriver until it comes to the Little Falls Dam and I'm pretty sure that above that there is no tide change. But the change at this cache location is at least like 30" between low and high tide which seems pretty darn significant. For reference, I looked up Sandy Pt, Maryland on the Chesapeake Bay and it's only a little over a foot there.
So the trick to finding this cache is that you need to pick your method and your timing wisely. The cache is accessible most of the time but it is much more easily accessible and also easier to find at certain times. As I already said, it is sometimes under water. So you need to go when it's not high tide and there aren't flood conditions. I definitely think the tide is much more of an issue than rain. I have been to the location by foot and by boat and they are both a fun trip. By foot is quicker but it is also more affected by the water levels. As long as you are within about 2 hours before or after low tide you should be pretty good. At dead low tide you can probably walk right to GZ on solid ground although it may be a little wet and muddy. Up to two or 2 1/2 hours away from low tide you can walk across the rocks if you've got ok balance skills. But start getting more than 2 1/2 hours away from low tide and you better pick another time or come by boat. The issue isn't that you couldn't just wade out there but WADING IN THE POTOMAC IN THIS AREA IS PROHIBITED AND YOU CAN BE FINED!!! So to make it easier for you I'm giving you a link to a tide table for the area. Just look at Chain Bridge and it will give you a good idea of when to go. You should have at least a 4 or 5 hour window around low tide on foot and a much broader window if in a boat. But just because you can get to the spot doesn't mean it's easy to find it when the water is high so do yourself a favor and observe the tide times and go when it is low.
https://www.tide-forecast.com/locations/Chain-Bridge-DC/tides/latest
As far as getting there by boat, I launched my kayak from Fletcher's Cove along with The Unstealthy Monkey who was coming along to test out the cache for me(thanks for that TUM!). Her kayak was IN her vehicle and mine was on top of my Civic which doesn't have a roof rack. That's important because you have to go through an old tunnel under the canal there and it is just about a 7' clearance. Maybe a ittle more at the center of the arch and a little less towards the sides. I read online that some people will park before the tunnel and carry their boat across the footbridge but it's probably about a .2 mile walk to the launch doing that. They also rent boats right there. If you bring your own you launch a bit downriver at the coordinates I provided. The current really was barely noticeable so going upstream was easy and it's about .8 miles to the cache.
It is a also a nice little hike that I had never done before but now I've done it several times scouting this out. It is just about 1/2 mile from the two parking locations near the bridge that I listed but those are not big lots. I've been a bunch of times and always gotten a spot but it was pretty much always a weekday. Make sure you follow the parking signs there and if it is full I am including two other parking waypoints to approach from a different direction where you can park on a neighborhood street and should have more space available. From the parking near the bridge, there is a trail that follows closely along the George Washington Parkway and if you want to do a little loop you can also follow along the shore of the river. No real trail there but it is pretty easy to follow. The trails are a little steep and rocky in places but they were nice enough to put a few handrails in at some of the really steep and rocky places. Hope you enjoy the cache and a nice little area that doesn't get a lot of visitors.