With the onset of winter upon us in Jackson Hole we complete another fishing season and mother nature gears up for it's annual snow pack buildup. The relationship between the snow pack and it's resultant river runoff as well as, for the purposes of this column, the fishing is worth having a better understanding of. It's a good opportunity to take a look back at the past year to see how things developed. Perhaps we will gain some insight into how we should view the future as a result.
First allow me to explain that rather than referring to the level of the snow pack in terms of the total precipitation, I prefer to use the snow-water equivalent because it is a more accurate representation of how much moisture is actually in the snow pack. When trying to understand how the snow pack will effect river runoff, fishing and all the rest, it doesn't really matter how high the snow is piled up, what matters is the amount of water in that snow pack.
Another concept to keep in mind...throughout this column, the snow-water equivalent is referred as a percent of normal for a given time of year. The data which is used to determine what is normal for a given time of year is collected by the Natural Resources Conservation Service. It is important to note that the reference period for average conditions is 1961-1990. So what we are really saying when we state that the snow-water equivalent in Jackson Hole was over 200% on January 5, 1997 (which it was) is this - the snow-water equivalent was over 200% of the average (or normal) snow-water equivalent on each January 5 between the years of 1961 and 1990.
Why am I pointing all of this out you may ask. It's because we have a tendency to assume that what we refer to as normal covers a much larger span of years than the 29 years used as the reference period (by the way, I don't know why the reference period ends in 1990). It means our concept of what is a normal snow year is quite limited in scope. The fact is, we really don't know what is a normal, or average, snow pack in Jackson Hole for any given date. The data collected by the Natural Resources Conservation Service is invaluable and we certainly should not ignore what it may indicate. The bottom line however, is that it is misleading to think that this data tells us more than it really does.
Having said that, lets take a look at how last winters snow pack came to be and what the results were. From this point in time last year until January 5, 1997 it snowed so much that we reached a snow-water equivalent of over 200% throughout the region. This alarming figure was the cause of grave concern on the part of some Jackson Hole "residents" who have built homes in the Snake River flood plain. Flood insurance was a hot seller at this time. Would the large snow pack lead to a 500 year (or worse) flood? It possibly could have, but it didn't. What it did lead to was an effort on the part of the aforementioned home owners to protect their investments by constructing levees along the river banks and other riparian areas. Levees are very destructive to the river and it's riparian environment and this was a very unfortunate and perhaps permanent side affect of last years snow pack.
The following two months were relatively dry and the snow-water equivalent dropped about 50% during this period. Still above 150% of normal, the snow-water equivalent was creeping down during a time of year when it very easily could have, in a "normal" year, rocketed upwards.
Starting sometime in early to mid-March and continuing for the next 8-10 weeks snow-water equivalents hovered around the 150% of normal level and by May 11 had actually increased slightly from March 16 levels. Soon things began to get interesting.
Snake River runoff began May 9 and by mid-May we experienced two weeks of fairly warm springtime temperatures for Jackson Hole. The river began to rise, quickly. The Bureau of Reclamation (the agency which governs water releases out of Jackson Lake Dam) had been increasing dam releases, thereby making room in Jackson Lake for flood control, prior to the onset of runoff. This was very fortunate and may well have prevented major flooding in Jackson Hole. Nonetheless, river flows, which are measured in cubic feet per second (cfs), reached levels which I don't think anyone in this valley can ever remember seeing.
The result? Well, no homes were flooded thanks to the Snake River levee system. There was, however, a dramatic alteration of various channels and flow patterns within the river basin. Though very hard to measure, I'd have to say we lost more trout habitat than we gained. At the very least, trout were in different locations than the year before, a result of their altered environment.
How did runoff effect the fishing? The Snake River typically becomes fishable for the fly fisher sometime between the middle of July and the start of August. This year it took a little longer and the Snake didn't fish all that well until mid-August. Anglers were out prior to that time, but didn't fare too well since the water was still somewhat off color and flows were still high. North of Jackson Hole in Yellowstone and south on the Green River conditions were more favorable than on the Snake and this is where most of the fishing pressure took place.
On the other hand, when water levels finally began to recede in September and October, angling on the Snake was quite good. While high river levels may not be conducive to high catch rates, the Fine-Spotted Snake River cutthroat trout rather enjoy and benefit from the extra water. It was a good growth year for trout and I wouldn't be surprised if mortality rates were lower this year than in the past, though this has yet to be determined. Numerous friends in the area have reported that this was a memorable October for fishing the Snake.
So there you have it. A huge snow pack, but no flooding. Fishing was tough early in the season but came on strong at the end. With winter just beginning certain questions inevitably arise. What will happen this year? Will the dreaded El Nino bring more or less precipitation? Can we derive anything from this years experience with which we can foresee what may happen next year? No way.