The USGS Tanner Rd gage has been around since Oct 2007. Not sure if another agency had a gage there previously. Here's the link: http://waterdata.usgs.gov/nwis/uv/?site_no=12141300&agency_cd=USGS

Looks like it got over 30k back in early 2009. And yea, House Rocks surely can move otherwise how would they have gotten there in the first place? As for what flow would be needed, probably no flow we'll ever see in our lifetime.

Edited by runawayjim - 07 Jan 2015 at 5:34pm

I think I can answer some of the questions, not about the Snoqualmie in particular but about streamflow.

"Is it normal with rain/snow melt events for the MF Snoqualmie to rise and fall as quickly as it did this week? Or did this rise and fall occur so quickly because we are a bit on the dry side of things this winter?"

There are two main sources to river flow: groundwater (water which moves through open spaces in the soil and bedrock) and surface water (water which flows on top, basically tributary streams and water sheeting down the ground surface). WIth regards to contribution to streamflow:

Groundwater contributes less in general to a river than surface water, particularly when the rivers are high. The contribution to groundwater also changes slowly, often seasonally, but it does go up a bit and then slowly come down after a rainstorm or snowmelt. When it's been a long time since rain or snowmelt, the water in the rivers is from groundwater; this is often called the 'base flow'. Also, in some instances the ground can actually soak up water from the rivers and even dry them out, if the ground around the river is dry. This is common in the desert, where mountain streams come into the valleys and get small or just plain disappear. Lots of small rivers in the US desert regions do this.

Surface water in a river comes from tributary streams mostly. It goes way up after rainstorms or snowmelt and then comes back down pretty quickly. It never takes away from streamflow like groundwater can in some instances. How long the high water spike lasts has to do with how long the rain or snowmelt event lasts, how big the watershed is (big watersheds are less flashy than small ones, and flat watersheds are also less flashy than steep ones), and whether the water was localized in one part of the drainage or not.

In this particular storm, I think a couple of things happened: 1) The storm was pretty quick and pretty heavy in the mountains (over 4" at Snoqualmie Pass), and 2) the air was warm so some mid-elevation snowpack was melted. The temps at the pass went from the 20s before the storm to the 40s after. [Data is here: http://www.nwac.us/weatherdata/snoqualmiepass/10day/] This means there was a lot of water put into the tributary streams in a short amount of time. There wasn't much lingering rain in the watershed either. The weather cleared up pretty fast. Then, factor in 3) river levels were pretty low before the storm (~800 CFS), which tells is that the contribution to base flow from groundwater is pretty minimal, because it had been pretty dry.


"How high have locals seen the Snoqualmie? Is there a way to look up past flows or find out about past flood events?"

On the USGS gauge page (http://waterdata.usgs.gov/nwis/uv?12141300) you can download all of the data using the buttons in the box sort of at the top of the page. You can have it make a plot of the data (stage or flow) for whatever time interval since the recording started (Nov 2007 for instantaneous measurements or 1961 for daily measurements), or you can select 'parms' which is probably supposed to stand for 'parameters' which basically means 'data' and have the data dumped to an html file or a text file, which you can maybe put into Excel or whatever to look at.

The other cool thing to answer your question more quickly is the little stats box below the discharge graph. It says that the maximum mean daily flow for 7 January over the measurement period (1961 to the present for this site) is 25,000, which is about the peak of this weekend's floods. So this is probably not an unheard of flood.

You can also play around with the graphs and datasets by clicking on the links on the gauge page. For example, here is a graph of the highest flows for each year: http://nwis.waterdata.usgs.gov/usa/nwis/peak/?site_no=12141300 The highest flow on here is 31,000 in both 2007 and 2009.

As to whether those flows can change House Rocks, I will have to cede to local knowledge but I would say that not the biggest rocks, but the little boulders could change which could impact how you the rapid is paddled. It would probably take a huge, huge flood to move the big boulder.






Edited by itchy - 07 Jan 2015 at 6:47pm

Not to geek out too hard on this, but as a hydrologist, I can’t resist…. 

Regarding groundwater vs. surface water contributions, generally speaking, most water coming from a moderate size flood event is actually “old water”, or water that was not from the current storm event. Instead, its water from a previous storm that has been trapped as interflow (shallow underground water) without sufficient gradient/head to reach to the stream. When a new storm comes through, it increases the slope of the interflow, and essentially purges the old water out, and replaces it in the shallow groundwater. So even in a flashy system, groundwater (specifically interflow) is often still a large contributor in stream flow. All this was discovered fairly recently (last 20 years) and really threw the hydrology field for a loop. 

With a really large storm, you see a stronger signal from the water entering the stream originating from the current storm, as soils saturate and more overland flow occurs (referred to as the variable source concept). For the MF specifically, it’s hard to say just by looking at Google Earth what the dominant factors are. It is a more flashy system than the Sky, but there are many factors beyond elevation and aspect to say with any certainty why the system is the way it is.

Regarding your last question, and Runawayjim’s comment, there are several ways in which boulders can enter a river, and with particularly large ones (like house rocks), it’s often the case that they got there not via the river (alluvium), but by a landslide/rockfall (colluvium).  So it’s not uncommon for there to be boulders that are too big for a river to move, even in massive floods.  House rocks are almost certainly colluvium; whether or not they're mobile at high flows is debatable. If they do move, its unlikely to move more than a few inches/feet in any given flood, but it's possible they’re not be mobile in any flood the MF watershed produces. That being said, betting against the river usually makes people look foolish, so I’d guess they do move from time to time. Of course you can get an empirical estimate of the flow needed to move on the boulders, but you need some data (slope, size/density of boulder, etc.), and I've wasted enough time at work already <> <> <>

where is JP when you need him? Sounds like you guys have no clue what you are talking about

Great discussion! The groundwater flushing stuff is really cool, I'd like to hear more about it.

To be clear, Herr Doktor Professor, my statements on groundwater and surface water were somewhat general and not specific to the MF Sno, as I stated in my intro sentence. Nonetheless, there are two ways to interpret Megan's questions: Is the MF Sno flashier than a typical drainage (yes, for reasons you've enumerated), and was this event in particular flashier than similar events in the MF Sno drainage (maybe?). The question of groundwater contribution to flashiness also affects interpretation 2: a groundwater deficit will definitely both limit the peak of the spike as some amount of the precip goes to recharge, and (I am reasonably certain) cause a faster decay in the streamflow post-peak as less water is seeping from the ground in the headwaters, and possibly recharge is still occurring post-peak.

I agree completely that the MF Sno is a surface water dominated system at very high flows (although groundwater is definitely non-negligible); the points that I've made are general and more applicable to sedimentary environments. Many a Southeastern paddler has watched September storms dump 3" on the dry watersheds only to see it all go down the karst drain, without a bump in the river levels whatsoever.

But I'm not a hydrologist (I just play one on the internet sometimes) so I could be wrong about the streamflow decay rate thing. I might do some data crunching, or better yet someone can find some studies.

Thanks for all the insight and helpful responses!  There is so much to learn about how rivers work!!

I really appreciated learning about all of this as well. Amazing Earth we live on!