Spring Snow

We awoke to snow cover on April 9; overnight precipitation left a couple of inches of fresh snow on the ground, blanketing lawns and other vegetation. Areas of pavement that receive the radiant heat of sunlight were mostly free of snow, but paved areas that remain in shade and otherwise protected from the warming rays of April collected two or more inches of fresh powder.

In our area, it's not entirely uncommon to receive measurable snow after Easter.

In our area, it's not entirely uncommon to receive measurable snow after Easter.

By the time April makes its way to the top of the calendar lots of folks around here are impatient for warmer weather. To them, snow in April feels like an event out of time. But we’re just two weeks into astronomical spring, the vernal equinox having occurred on March 20 this year, so it didn’t strike me as particularly strange to see snow on the ground. I was, however, curious about past patterns of snowfall, so I checked with the National Weather Service (NWS) of the National Oceanic and Atmospheric Administration (NOAA) to obtain some “Late Season Snow Climatology” information.

It is worth noting that when meteorologists use terms like “normal” or “average,” they usually mean a 30-year average. Here in Rockford, for example, the current NWS “normal” is based on records compiled between 1981 and 2010.

According to the NWS, the “normal” last date of the season on which to expect more than an inch of snow is March 20. However, the “normal” last date for a trace of snow is April 11. So the snow we received this week can hardly be seen as highly unusual. Looking back at earlier records, the latest “last measurable snow” in Rockford was May 11 (1966), and that was more than an inch of accumulation.

I was naturally curious about the phenology of the first snows of the season, as well. It turns out that the “normal” first date on which we receive a trace of snow is November 2; to receive more than an inch we typically would have to wait until December 5. I was surprised by the earliest first date of a trace of snow (October 3, 1951) and equally surprised by the latest first date of an inch or more of snow (February 7, 1921).

On the one hand, we could have snow around here any time between early October (as on October 3, 1951) and late May (like May 24, 1925). Yes, that’s 8 out 12 months during which we might see snow. On the other hand, our first trace of snow might not arrive until the latter half of December (which it did on December 19, 1999) and the last trace snowfall might be over in early March (as it was on March 5, 2012).

Climatologists recognize that weather patterns often vary tremendously year-to-year or decade-to-decade. They have to look at long-term patterns to gain an appreciable understanding of climate.

And this is why we collect phenology data for local plants, recording the dates of events like breaking leaf bud in the spring and colored leaves in the fall:  every year is different. And with every season we witness the cycle of events, turning, spinning, spiraling around and through our perception of the natural world.

Tale of the Thaw, Part 2

Morning mist lifts from fence lines drawn between farm fields draped in melting snow; ghostly shrubs, veiled in frigid fog, lurk behind the glistening vapors of dawn.

Such were the scenes of the second thaw of the month as mild temperatures returned to the area during the latter half of February. The snow and ice that had accumulated over previous weeks quickly melted and flowed to our watercourses. First-order streams (the smallest streams under current stream classification systems, also known as headwater streams) are the first to swell with meltwater. These tributary stream channels fill and flow rapidly toward larger waterways, with water volume and velocity increasing as temperatures above freezing continue to contribute matter and energy to the system. Higher-order streams—like our rivers—accept the contributions from many tributaries; their levels crest as water levels in low-order streams subside.

All of this makes for some dramatic scenes around the area. Here at Severson Dells, one of the salient features of the landscape is the first-order stream that we call Hall Creek. It has its origins north of Montague Road and enters our forest preserve from the west, meandering through many switchbacks on its way to the Rock River. When the thaw began I went out to see how rapidly it would fill. On February 19 it was flowing freely. On February 20 it had overflowed its banks. And this was before the Kishwaukee or Rock Rivers had risen appreciably.

Left:  Hall Creek begins to rise, February 19, 2018. Right:  Hall Creek in flood, February 20, 2018.

Left:  Hall Creek begins to rise, February 19, 2018. Right:  Hall Creek in flood, February 20, 2018.

A healthy, resilient waterway can accommodate flooding. The curves and bends of a meandering stream serve to limit the velocity of the water as it flows. The floodplain along the banks can accept the overflow, gently ushering the excess water downstream. The natural vegetation of the floodplain helps to protect the soil from erosion. Even during the dormant season, the roots of those plants are holding the soil.

The resilient functionality of Hall Creek was in evidence on February 20, the second day of the big thaw. Floodwaters overran the channel of the creek and spread out onto the frozen floodplain. The creek ran through braided channels and flowed over new terrain. The watercourse carried meltwater that might otherwise have found its way into basements or across roads. Unlike this stretch of Hall Creek, many of our streams have been channelized, ditched and straightened in an effort to increase tillable acreage and improve productivity—and to whisk away excess water as quickly as possible. Unfortunately, that engineered efficiency comes at a cost:  all of that fresh water is taken away when it might otherwise have been returned underground to recharge our fresh-water aquifers. Groundwater recharge is an important function of wetlands like the floodplain forests and riparian marshes that line healthy streams. Ditched channels with steep banks hasten erosion and carry sediment, rich in organic matter, rapidly away to places where the silt is likely to do more harm than good. The meanders of a natural stream like Hall Creek slow the flow of water, reducing erosion along the banks and allowing more water to go underground.

The area in the foreground is usually well above water, with Hall Creek confined to a channel behind. The floodplain deftly carried the excess water downstream toward the Rock River.

The area in the foreground is usually well above water, with Hall Creek confined to a channel behind. The floodplain deftly carried the excess water downstream toward the Rock River.

Before the first wildflowers of spring open to the warming sun, our streams come alive, dancing with water drawn from melting snow.