Fly Fishing and the Flow Dynamics of the South Platte River Basin
The South Platte River Basin is a FlyCast favorite as it offers incredible fly fishing and flows through a diverse landscape all within driving distance of the Denver Metro area. Here you’ll find a wide range of trout species and have the opportunity to catch some of the state’s largest trout. Not only is it an outlet for anglers to immerse themselves in nature, but it is a crucial water source for front range city dwellers. While understanding current flows on a given section of the South Platte is critical to success when fly fishing, the system is highly interconnected. Understanding the flow patterns among the broader system will allow you to predict changes among the various fishing locales and set you up for success on your next outing.
When it comes to flows across the basin, understanding the path and various dams and reservoirs is critical. There are two primary segments of the South Platte, the North Fork and the South Platte (proper). Both of which play a critical role in supplying downstream water demand.
North Fork: The North Fork originates southeast of Silverthorne, picking up water from a number of tributaries and from Dillon Reservoir via Roberts Tunnel before making its way east to Strontia Springs Reservoir. Strontia Springs Reservoir is a key landmark for the South Platte as it is where the North Fork and South Platte (proper) meet up before making its way to Denver. The North Fork is home to a number of great private water sections, namely North Fork Ranch, Boxwood Gulch and the Meadows.
South Platte: The South Platte (proper) originates in Hartsel, Colorado, where the South Fork and Middle Fork converge to form the headwaters. While Antero (derived from the Spanish word for “first”) marks the first notable water impoundment for the South Fork, Spinney Reservoir, east of the headwaters, is the first major stop for the South Platte system. Beyond that, the river runs several miles southeast to Eleven Mile Canyon Reservoir before turning north to Cheesman Reservoir, south of the town of Deckers. From there, the South Platte flows northeast towards the North Fork confluence. Several miles downstream of the confluence, the South Platte spills into Strontia Springs.
Now that we understand the flow path and the major water bodies of the South Platte Basin, we can start to conceptualize both seasonal trends and day-to-day flow patterns. While the Colorado Department of Wildlife considers the impact of flow on trout habitat and conservation, at the end of the day it all comes down to water demand in the Denver Metro area. Denver Water, Colorado’s largest water utility, can effectively call on water from the South Platte in order to meet customer demand.
Seasonal Trends & Big Picture
Broadly speaking, Denver Water sources water from both the North Fork and the South Platte (proper). However, they systematically manage flows so as to ensure ample reserves across the basins’ many reservoirs. In general, it is common to see the North Fork running high while the South Platte (proper) is running slightly lower. Meaning, Denver Water is sourcing most of its supply from the North Fork. That said, this is a generalization and not always the case. For example, Denver Water is currently running the North Fork High in order to meet demand. At the same time, Cheesman Reservoir, which is one of the last stops before reaching Strontia Springs, is unseasonably low. So in an effort to replenish water stockpiles, or reserves, at Cheesman Reservoir, Denver Water is pulling a high volume of water from Spinney Reservoir and Eleven Mile Canyon Reservoir in anticipation of making the transition to source water from the South Platte (proper) in the fall when Roberts Tunnel undergoes construction.
Day-to-Day Flow Patterns
From a day-to-day perspective, it is a little more straight forward. If flows were to increase or decrease on the Dream Stream, it is likely that you will see a similar reaction in Eleven Mile Canyon several days later. While this is not always the case, as it depends on water levels downstream of the Dream, it is a good rule of thumb. Similarly, if you were to see flows increase or decrease in Cheesman Canyon, you will see an immediate change through Deckers.
Why is this important?
Understanding seasonal flows allows us to get a longer term view on how certain sections of the South Platte will be fishing and allow us to plan accordingly. For example, we mentioned that Denver Water is working to replenish inventories at Cheesman Reservoir through September. This means flows will be low in Cheesman Canyon and Deckers this summer. Low flows, coupled with summer heat, means that fishing can be tough as trout will be sluggish with high water temps. Knowing this, it is best to fish these sections in the morning or evening from both a productivity and conservation perspective. Conversely, you might consider fishing the Dream Stream or Eleven Mile Canyon knowing that flows here will be comparatively higher.
When it comes to more dynamic (i.e. day-to-day) swings, changes in flow can mean a number of things depending on the direction and starting point for which they started. Assuming flows were comparatively low at the Dream Stream and we saw a moderate bump, this is a signal to get out and fish as trout will have more feeding lanes/holding positions as well as increased aquatic bug life leading to higher productivity. That being said, you’ll want to give the trout a day or two to normalize to this new flow. Conversely, a decline in flow doesn't always mean that productivity will suffer. In fact, trout actually have an easier time adjusting to lower flows.
Being able to anticipate and understand both the seasonal and day-to-date flow dynamics across the South Platte will dramatically improve your game. While it is not completely necessary to do so, it will give you a competitive edge against both trout and your fellow angler.