Knowing the flows of the water you intend to fish is always a good idea. There are a number of factors that influence flows and when the water is too high or low it can really impact the fishing. Time of year can make a big difference on the water levels and flows. Spring run-off and late summer low water are two things you can expect and be aware of. In addition to these, be aware of the release from dams in tailwaters, if that is where you are headed. Personally, I have had trips to the river fouled by not thinking about the impact of water release ahead of time. Keeping these things in mind ahead of time can payoff and save you some wasted time. To check current flows there are a number of online resources available to help you determine your best option.
Another important factor to keep in mind when heading out to fish is what type of insect activity you can expect to find when you get there, and what flies to bring with you to be prepared. A lot of this is predictable and if you are familiar with a particular body of water you can plan accordingly. Knowing the water you are fishing and preparing for that is going to be an important factor in whether your fishing is successful or not. Having this basic understanding of which insects it is reasonable to expect to be around and to plan accordingly is important when fishing a specific piece of water. Weather and time of day are also some things to consider when heading to the water. These variables can have a large impact on what bugs are coming off and thus on your fly selection. In order to give yourself the very best chances of having a great day on the water do a little research on the piece of water you are heading out to, whether an old favorite or a new spot, and arm yourself with the right flies for that river. If you aren't sure where to start your search for this information, a call to a local fly shop is always a great place to get the latest information as to what is happening and some good suggestions for what patterns to be throwing.
Weather is also something that you want to pay attention to before you head out. Knowing recent and current conditions can both be useful. If there has been recent rainfall in the areas you want to fish you need to know that the water clarity maybe an issue. In terms of current conditions, this can seriously impact what flies you will be using. Depending on the weather for the day, a body of water can fish a number of different patterns and approaches well. In addition to light and precipitation conditions, the wind is also something to think about before choosing a spot to fish. Wind can really be something difficult to work around on the water, and knowing this can help you narrow your fishing options. Learning how the weather impacts fishing and being prepared for something we can't control is of great value.
In all areas of life it is good to be prepared, and the fishing is no different. Learn as much as you can about your waters routine behaviors and external factor's influence and you will be better off for it. The same goes for destination fishing. Find out all you can about the water and area you are traveling to and it will pay dividends when you are out on the water.