Truth is, it's a little of both. The more you know about the lake and about the fish you seek, the better the odds of filling your limit. Once armed with the knowledge, the art of locating fish and enticing them to strike is the task for the day. Now for good measure, let's add another factor to the equation... Skill. Yes - the skill of detecting strikes, setting the hook and landing the fish.
Exactly What Is A Lake?
Most lakes and reservoirs are created by placing a dam in a waterway. As it fills, it floods acreage on both sides of the waterway up to the elevation of the spillway on the dam. The deepest part will always be the old waterway - a riverbed, creek bed or wash.
Fish use the old riverbed as a highway to navigate their way around the reservoir. Other creek beds running into the lake also be come highways for transit. Fish seek out an area of a lake that has good water quality, food and cover plus a variety of depths in close proximity. This area will have a deep holding area (generally in or along the riverbed), a medium depth holding/feeding area and a shallow feeding area. All three of these areas must provide cover and escape routes to deeper sanctuaries if needed. PH can be a factor but only in extreme conditions. Barring changes in conditions most fish will live out their life in this relatively small area. Actual depth will depend on the species, reservoir configuration and water conditions.
The best way to approach a new fishery is to learn as much as you can before you leave home. Look for articles, fishing reports and fishing tournament results at that lake. The internet is loaded with information for those willing to track it down.
Get a good fishing map, the more detail the better. Get a lay of the land to determine where the deep channels are. Locate large flat, shallow areas. Identify the major coves or arms of the lake, ones with a significant river channel or creek bed.
Plan of Attack
If the lake is too big to cover in a day you can focus your efforts on a small sector in order to define what the fish are doing. Select a major cove define a pattern of fish location for the day. This cove will be a miniature version of the lake. The creek bed in the back of the cove is similar to the major riverbed feeding the back end of the lake. The back of the cove is shallow like the river end of the reservoir. The water gets deeper as you move out to the main body of the lake just like the lake does as you move toward the dam.
Cover all your options. By fishing various stations of the cove you can determine where the fish are holding. At each station, fish deep, then medium depths and then shallow. Use lures you can cover a lot of water quickly.